How could a company that only had about $60 million in revenue—according to an investor pitch deck obtained by TechCrunch —be worth anything close to $20 billion? That must be evidence of a bubble. That would mean Snapchat is worth more than 250 times its revenues, when Facebook is only worth about 17 times its annual revenues.

Calculating a company’s valuation is about a lot more than just plain mathematics, however. One thing that matters more than almost anything else is the growth curve a company is on. If it’s a social app, which relies on network effects, then it’s all about active users and engaged time and the growth of those things, and only tangentially about revenue. Or rather, many investors are happy with revenue being an estimate.

That’s why there is such a huge difference between the theoretical valuation of Snapchat and the valuation of Twitter (TWTR), which has about the same number of daily active users—100 million—but is only worth about $10 billion. Snapchat’s user base grew by more than 50% last year, according to its pitch deck, while Twitter grew by such a small amount it’s barely worth mentioning.

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In other words, investors would much rather have a piece of Snapchat’s growing user base of 110 million daily active users than they would a piece of Twitter, even though Twitter’s revenue last year was $2.2 billion, more than 35 times what Snapchat made.

Then there’s the fact that Snapchat didn’t even start trying to make money from its users in earnest until the last quarter of 2015, when it started introducing “sponsored lenses.” So the bulk of the $60 million it made last year came in the fourth quarter. That’s why the company is projecting revenue of between $250 million and $350 million for this year, and as much as $1 billion in revenue for 2017—which would make its current valuation 20 times revenues.

Obviously, those numbers are just guesses. Snapchat could introduce a variety of new ways to make money that users hate and they might reject the app in droves, or advertisers might not like the prices that the company is charging and they might have to discount them dramatically. Or, the company could come up with more sponsored filters like the Taco Bell one, which was viewed a mind-boggling 224 million times.

Snapchat just raised almost $2 billion in funding. Watch:

What we know currently is that Snapchat’s revenue appears to be growing rapidly, and that it will likely do $300 million or more this year, if not more. And we know that its user base was growing at a more than 50% rate last year. Surveys done by firms like Piper Jaffray also show that the app is the number one choice for social media among teens, well ahead of Instagram, Twitter and Facebook, and users spend as much as 30 minutes on it every day.

Earlier this year, Snapchat said that its users are consuming more than 10 billion videos a day, a number that puts it ahead of the most recent statistics about video on Facebook. Regardless of whether those views are directly comparable to the audience a TV show gets, that is a massive amount of attention.

Valuations of Facebook (FB) in its early days were similarly difficult, because the network spent all its time building up a base of users and trying to make the site as sticky as possible, and only then did it start to try and make money. But when it did, the amount of revenue it generated was staggering. So is Snapchat worth $20 billion? That’s difficult to say — but it’s easy to see why some venture investors think that it could be.