Instagram is hanging on to a demographic that’s rapidly abandoning the Facebook platform, according to a new RBC Capital Markets survey, making Mark Zuckerberg’s buy of the photo-sharing app “one of the best acquisitions in internet history” according to one analyst.

“We clearly see a fade amongst the 13- to 18-year-olds in usage of Facebook, and I think most people would probably intuitively feel that that’s happening,” Mark Mahaney, lead internet analyst at RBC, told CNBC’s “Power Lunch” Monday.

73 percent of survey respondents between the ages of 13 and 18 had an Instagram account. Only 57 percent had a Facebook account.

Both social media platforms dropped in usage among that youngest cohort in the six months preceding the report, but Facebook took a much larger hit than Instagram. In June, RBC found 81 percent of 13- to 18-year-olds had Instagram accounts and 75 percent had Facebook accounts.

Instagram, then, is “the teen and millennial hedge for Facebook,” Mahaney said.

The RBC survey is relatively new, now in its third six-month iteration, so it’s not yet clear from the survey how users grow into our out of particular social media platforms. Mahaney said it’s possible those 13- to 18-year-olds will get older and return to the Facebook platform.

“It’s going to be like a utility. People like utilities — there’s no emotional attachment to a utility, but you’ll use it as a utility to keep in contact with extended family and friends,” Mahaney said. “But the hearts for young people clearly are with Snapchat, the hearts clearly are with Instagram.”

To be sure, Facebook still far outpaces Instagram, Twitter and Snapchat in percent of U.S. internet users with an account.

According to the survey, 91 percent of all U.S. internet users have a Facebook account. Instagram comes in at only 47 percent of all U.S. internet users, Twitter at 42 percent and Snapchat at 31 percent.