Facebooks business is ads. Facebooks largest growing market is the third world. Ads are worthless there because people have no money to spend on sh*t they don't need. Now if FB makes micro transaction possible and cheap and even available for the unbaked, suddenly the poorest of the world are potential customers and thus can be targeted with ads. They may only have fractions of dollars to spend but they are literally billions so it ads up.

TL;DR Banking the unbanked with cheap micro transaction makes them milkable.