If you just come out of a cave (or exhausting revisions), it is possible but unlikely that you've ever heard of Agar.io. This is the origin of a game only available on browser, in which players control a tiny ball. The goal then is to make it grow, first by drawing tiny dots scattered throughout the map, then, once acquired a certain size, absorbing the balls of other players, as is a Agar.io online game. Three additional small subtleties are grafted to this simplistic gameplay.
First, the greater the ball grows, it becomes slow, so that it can not swallow smaller too easily. When we know that larger objects can easily take a tenth of the game space is something important. The player then has two special powers. The first allows you to split his ball in two, so take squeezed a ball from another player. The second is used to project the small bubbles to either lose weight to go faster or capture enemy too fast balls. The whole gives an extremely addictive game, with the limit of the hypnotic.
His mobile porting is no surprise. The principle is identical in every respect, except that it is played with touch controls. These are relatively easy to learn, but however prove somewhat less accurate than a mouse on a real good old PC. For now it is not possible to connect to the game via Facebook, or choose its servers. Little is known whether one plays only with other players or mobile Agar.io on Android is cross-platform.
Agar.io is free to download but appears from time to time advertising videos, there is also hack for agar.io here. The listing on the Play Store also indicates that in-app purchases are present in the application but I have never found any trace of a shop or a purchase in the game. It is likely that the developers integrate in the future.