Pocket Gems, the Sequoia Capital-backed casual mobile gaming company that has three of the U.S.’s top-grossing 11 apps, finally made its debut on Android.

It brought one of its older hits Tap Zoo to Android Market and has gotten at least 10,000 downloads so far — presumably much of it through paid marketing channels since the company doesn’t have a pre-existing userbase on the platform. Initially released last year, Tap Zoo is the title that helped Pocket Gems really establish its network and break past 40 million downloads as of last month. It also spawned numerous similar titles like Storm8′s Zoo Story and TinyCo’s Tiny Zoo, which has been performing equally well on iOS as of late.

Many of iOS’ biggest freemium gaming companies are making the leap to Google’s platform to join rivals like Storm8, Com2Us and Glu Mobile, which already have established audiences there.

With Android seeing north of 550,000 device activations a day and in-app billing, the platform is finally starting to produce a meaningful amount of revenue for some developers. Gameview Studios, which makes Tap Fish and was acquired by Japan’s DeNA last year, said the gap in average revenue per user is narrowing between the two platforms.

The upper bar for revenue from in-app purchases on Android seems to be slightly more than $1 million a month for a single game. That’s what Game Insight, the Moscow-based company behind the top-grossing game Paradise Island, told us its earnings were last month. While that’s a fraction of what a comparably ranked game would make on iOS, it’s nothing to sneeze at since top-earning iOS games were on a similar monthly runrate in 2010.