Start-ups valued at $1 billion or more used to be so rare that they were dubbed “unicorns.” Now, they number 215, according to CB Insights estimates.

That’s no coincidence, according to LinkedIn co-founder Reid Hoffman, who spoke to CNBC’s Jon Fortt this week.

Public markets have made it clear that a successful technology platform is worth more than almost any other business. Just look at the market capitalizations of Apple, Alphabet, Facebook and Amazon: $826 billion, $647 billion, $490 billion and $460 billion, respectively, according to FactSet.

That means when it comes to setting start-up valuations, “it’s very hard to make that call intelligently,” Hoffman said as part of the “Fortt Knox” podcast.

“One of the thing’s that’s been interesting is if you contrast the top five most valuable companies now, versus 10 years ago: 10 years ago it was more oil companies, now it’s more tech companies,” Hoffman said. “Because they benefit from this kind of global scale, in a networked world, like oil companies have always benefited from a global scale. So then you get to all these other valuations.”

Hoffman has advised companies like Microsoft, Mozilla and Airbnb. His pedigree also includes leadership roles at Apple and PayPal.

Now he’s an investor at venture capital firm Greylock, which invested in such companies as Pandora, Facebook and Instagram.

“The question is, well, are they going to be one of these huge, gi-normous companies that benefits from the networked world?” Hoffman asked. “So if you say, there’s a high probability that they are, then they’re probably undervalued; …. low probability, and they’re probably overvalued. Now, what it does mean is that most of the valuations are probably too high, right? “

The sky-high valuations in Silicon Valley have left some investors jittery over the past few years. Benchmark general partner Bill Gurley, for example, has warned that “loose capital” has allowed reckless, sloppy entrepreneurs into the playing field. Mark Suster, managing partner at Upfront Ventures, warned that unicorns were breeding a rotten culture in the Valley that would outlast the market’s boom.

But Hoffman said finding the right pricing for a deal is just part of the challenge of investing.

“Only a small number will define the next platforms, the next major tech companies, and so forth,” Hoffman said. “And so on that risk-adjusted bet, if you said I took a portfolio of 100 of them, probably 90 or something are overvalued. And 10 are undervalued. Or five are undervalued. And that’s the challenge for being a professional venture capitalist and investor and so forth, is trying to discern those companies.”