Snapchat is estimated to have 170m users

"Now you see it, now you do not. The original appeal of Snapchat, a social messaging app, was disappearing photos that vanish after 10 seconds or less. There are plenty of ephemeral apps, but Snapchat is the biggest. Less than four years old, it has 170m users (Cowen estimates)- two-thirds as many as Twitter. And it is fending off investors who would buy in at a valuation of $16bn or more.
Snapchat is valuable not only because of its fade away photos. Snapchat has messaging features (for text, photos and video); socially shared posts; and even a professional news platform. Time spent by US teenagers aged 14-17 on the app rivals Facebook (30 minutes per session on Snapchat, versus 34 for Facebook, according to Magid Associates). Its young, engaged audience, most users are under 30, makes it popular with advertisers.
Whether Snapchat itself might be as ephemeral as its photos remains to be seen. It has grown fast. Snapchat has five times as many users as Facebook did at the same age (3.5 years old). It has also been faster to commercialise: Cowen estimates the app had $200m in revenues this year, mostly from ads. But it is early days; it could disappear just as quickly (remember MySpace?) Already users bemoan a clunky design as new features clutter the interface.
None of this has stopped investors. A $16bn valuation for Snapchat, roughly $90 per user, would be on par with Twitter ($95 per user), though less than Facebook ($145 per user). These valuations are as much a reflection of the cash sloshing around the tech sector as of real users and profits."

Source:  FT Lex, 21st February 2015

Note - Cowen seems to be The Cowen Group - but there's no information on whether the estimate is MAUs (monthly active users) or total downloads