Three years ago Dyan deNapoli, a 57-year-old author and TED speaker who specializes in penguins, was given a 23andMe genetic testing kit for her birthday. Intrigued, she spit in the tube and sent the results to a lab in Burlington, N.C.

About two months later she received a pie chart breaking down where her ancestors lived (99.4 percent of them were from Europe). What she was most giddy about, however, was a 41-page list of all the people who had done the test and were genetically related to her: 1,200 in all. (Customers can choose whether their information is shared with others.)

“I had the names of everyone from my immediate family members to my first cousins, second cousins, third. Once I got past fourth cousins, it went to my fifth cousins, and beyond,” said Ms. deNapoli, who lives in Georgetown, Mass. “It started me down this genealogical rabbit hole.”

Using the website’s internal messaging system supplemented with Facebook, she connected with three second cousins, who were in neighboring towns. She met each one for breakfast in a local diner, where they spent hours drinking coffee and poring over family trees and photos, marveling at various resemblances.

“Jorge is an older cousin, a very young 90,” Ms. deNapoli said. “Everybody agreed he looks just like my dad.”

Last June she visited a third cousin and other relatives in a mountainous village in the Campania region of Italy, her paternal grandmother’s place of origin, walking the narrow streets, eating four-course meals and learning stories of her ancestors, including a long-ago Hatfield-McCoy-level feud. “That’s why I really didn’t know this side of my family,” Ms. deNapoli said in wonderment.

At-home genetic testing services have gained significant traction in the past few years. 23andMe, which costs $99, has over five million customers, according to the company; AncestryDNA (currently $69), over 10 million.

The companies use their large databases to match willing participants with others who share their DNA. In many cases, long-lost relatives are reuniting, becoming best friends, travel partners, genealogical resources or confidantes.

The result is a more layered version of what happened when Facebook first emerged and out-of-touch friends and family members found one another. Children of long-ago casual sperm donors are finding their fathers. Adoptees are bonding to biological family members they’ve been searching for their entire lives.

Sherri Tredway, 55, is a marketing and development director for a social service agency based in Washington, Ind. She was adopted as a baby, and in January she drove two and a half hours to Bowling Green, Ky., to meet her biological half sister, Patty Roberts-Freeman, 60, with whom she connected through AncestryDNA.

Ms. Roberts-Freeman needed an outfit for a wedding, so they arranged to meet at a shopping mall to find one together. They started in the food court, where they bought sodas and talked for over an hour about their mother, their current lives, their upbringings.

They then went to a Belk department store, where they tried on outfits. “I was looking at some dressy dresses and showed her a few, and she said, ‘No, no dresses for me!’” Ms. Tredway recalled. “I remember saying, ‘O.K., are you sure you are my sister?’ which we both laughed about. She found a silky floral shell and a beautiful sweater in rose, pink and cream to wear with some slacks. It was very classy.’”

The half sisters have since seen each other several times, meeting in restaurants between their homes. They also see other relatives including two more half siblings, Sissy Bonham, 51, and Michael Clavette, 54, as well as their biological mother’s sister, Nancy Kalman Bell.

“Not a day goes by when I don’t talk to Aunt Nan,” Ms. Tredway said. “I call her to talk, when I’m upset, anything. She’s my family now.”

Josh Broadwater, 44, a deputy police sheriff in California, was abandoned when he was 1 day old in a gas station bathroom in California. When he was in his 30s, he implored the agency that placed him with adoptive parents to give him whatever information they had about his biological ones but ran into constant dead ends.

In July 2015 he sent a kit to AncestryDNA and found a cousin who shared DNA with him. That led to him discovering his biological father: a man who had had a one-night stand in the front seat of a ’69 pickup truck and never knew he existed. They connected over the phone, and soon Mr. Broadwater was driving 500 miles to go elk hunting on his father’s farm in Kingston, Utah.

“He kind of sat there quiet for 10 to 15 seconds,” said Mr. Broadwater about their first conversation. “And then in his cute little country voice he said, ‘Well, if Gloria is your mom, and this thing says I’m your dad, there is a damn good chance I am your dad.’ He is just the coolest person.”

The two got along so well that they now talk on the phone once a week about the weather, what is going on with the children, about the hunting season. “I never thought finding my biological dad, he would be the one calling me,” Mr. Broadwater said.

He also talks to a half brother who is eight months older than him. “I just got a text message from him that I’m going to be an uncle in October,” Mr. Broadwater said. “I don’t know how much I will be involved. This whole new family is new to me.”

Others who have their DNA tested are forming relationships not with specific people, but with their family’s places of origin.

One example is Leah Madison, 32, an education outreach coordinator for the Desert Research Institute in Reno, Nev. She was planning trips to Peru and Korea when she learned a year and a half ago from 23andMe that her family came from Greece, Italy and the Iberian Peninsula.

Over the winter she and her father went to the Iberian Peninsula for two weeks. She felt an ineluctable connection to the people as she ate their bread masterpieces, toured buildings by Antoni Gaudí and danced to flamenco music.

“I had a piece of paper that tells me I’m from Spain,” Ms. Madison said. “But then I went there and I noticed all these people have curly hair, and maybe that is where mine comes from?” Now she feels compelled to visit the other places as well.

But other testers have found their results more alienating.

In February 2016 Christine Carter, a marketing strategist, was on a business trip to London when she decided to open her 23andMe dossier. She was in her hotel room, rushing to dinner. “I thought it would be a quick reveal,” she said. “I was going to learn that I was Native American and black, and maybe learn a little bit more about the stories I heard as a child.”

Ms. Carter was shocked when the results showed she was 31.5 percent white or European. She struggled through dinner, keeping this revelation mostly to herself, until she got back to her home in Baltimore and contended with her feelings.

She wrote a Huffington Post blog post, “I Celebrated Black History Month … By Finding Out I Was White” that went viral. It attracted thousands of comments, from white supremacists who berated her, to people who had a similar experience and shared her sentiments.

“It took me less than 30 minutes to write the post, it was like journaling, something to get it off my chest,” said Ms. Carter, 32. “So to have that reaction was insane.”

Perhaps the most frustrating reality is when users don’t have any known connections at all. This can happen to people in certain ethnic groups, including Latinos and Asians, that thus far have fewer people using the services and a smaller database.

“Diversity in genetic research is a global problem,” said Joanna Mountain, the senior director of Research at 23andMe, adding that the company is offering free testing in some countries to begin to rectify that. “The results for Hispanics and Asians aren’t there yet, but they are coming,” said Jenn Utley, a family historian at Ancestry (the parent company of AncestryDNA). “The database keeps growing.”

Even for those privy to rich data, using a genetic-testing service as social network poses challenges. Ms. deNapoli has written to 25 people related to her and has heard back from only nine. “I guess a lot of people aren’t doing the tests to connect with family,” she said.

Ms. Tredway said she had a difficult experience after reaching out to her biological mother, getting an out-of-the-blue phone call from North Carolina while she was getting a haircut: “She said, ‘There is no way you could be my daughter,’ even though I knew I was.”

And then there is David Hughes, 38, the owner of an executive search firm, Sandbox Partners, who was ecstatic when he got his results back from 23andMe. “My breakdown is basically 60 percent Balkan, which is Mediterranean or Greek, 25 percent Native American Indian, 11 percent Middle Eastern and 4 percent Eastern African,” he said. “I’m like the heritage of warriors or something.”

But as much as Mr. Hughes wants to explore the different regions he comes from and meet the family members whom he got that DNA from, he hasn’t matched with anyone through the genetic testing service.

“My biological dad is 50 percent Native American Indian, so I eventually hope to find which tribe I am from,” he said. “But I have nothing yet.”