Remember Paper Toss? Of course you do. Alongside games like Tap Tap Revenge and FieldRunners, it was one of those first iOS game releases that really made a mark on the App Store – long before the days of disgruntled birds and hungry monsters. Paper Toss developer Backflip Studios has gone on to develop a myriad of great free-to-play titles since then, and has now announced one heck of a milestone: Backflip Studios has accrued more than 100 million downloads since their launch less than two years ago.

Offering games for both iOS and Android, Backflip says that 100 million is broken up as a roughly 80/20 split – that’s 80 million iOS downloads versus 20 million on Android. Of these 100 million, the most popular have been Paper Toss, Ninjump, Paper Toss: World Tour and Ragdoll Blaster 2.

A few other interesting numbers were released as well, providing deeper insight into Backflip Studios’ success to date. The developer has revealed that their 2011 revenue will be generated evenly between paid game downloads, mobile advertising, and in-app purchases. Diversification, it would seem, is key.

Backflip is also claiming some surprisingly high user retention numbers, sporting 25 million monthly active users and 2.5 million daily active users across their catalogue of games.

So what’s next for Backflip Studios? This morning kicked off their second annual “Summer of Free,” in which a series of free-to-play titles will be released to iOS and Android gamers.

First up is an iPad version of their popular shuffleboard-meets-bowling game Strike Knight, which is available today. After that expect to see Shape Shift on June 16th, a puzzle game that the studio is billing as “our next Paper Toss” and their biggest release of the season. This will be followed by Backflip Solitaire in July, and Boss Battles – a top down vertical shooter starring a racoon bounty hunter – in August.

The studio is also promising three additional free games that will be added into the mix sooner than later, as well as a paid version of Ragdoll Blaster 3 arriving in September.

Want to know more about Backflip’s upcoming summer line-up? Be sure to follow these games right here on Gamezebo to be notified the moment we post up a preview!