Facebook makes 5-year deal with FarmVille company Zynga
The press release is very vague about the terms of the agreement, and it doesn't mention whether any money changed hands, or whether Zynga will get privileges other developers don't have. It's clear that both companies have to keep one another happy, because Zynga games are played by over 200 million Facebook users, and Facebook is (nearly) Zynga's entire business model.
According to the most specific part of the release, "Zynga is currently testing Facebook Credits in select games and will expand to more titles over the coming months. Terms of the agreement between Facebook and Zynga were not disclosed."