So maybe that $1 billion it cost Facebook to buy Instagramwas a bargain after all.

The photo-sharing app is on fire. It now has more than 50 million users — and is adding new ones at the rate of roughly 5 million per week, according to its API.

Instagram hit 40 million users around April 13, and 30 million at the start of the month (no April Fooling). This is astonishing growth for an app that started 2012 with just 15 million users.

Of course, the fact that the app was recently released for Android — after being iPhone-only for its first two years — is likely driving a lot of those sign-ups. But Instagram also hit number one in the iOS App store for the first time in the wake of the Facebook acquisition.

Google doesn’t break out exactly how many Android downloads an app gets on its Google Play page. But Instagram did cross the 10 million download threshold two days ago, so it’s a safe bet that roughly 40 million of those users are on the iPhone.

In other words, iPhone and Android have divided Instagram sign-ups fairly equally since it became available for both platforms. (So much for Apple executive Phil Schiller’s claim that the app had “jumped the shark.”)

And who is Instagram user number 50 million? According to Gramfeed, it’s a user named fox289. All we know about the user is drawn from his or her profile picture: an FC Barcelona soccer shirt. Just how many accounts are like fox289 — with no photos posted, no users followed — is something Facebook will be very interested in.

-mashable.com