They aren't just throwing dung at the wall and seeing what sticks: they're very, very good at what they do. For every FarmVille there are tens of thousands of games which you have never heard of: their viral growth coefficient was .95 instead of 1.05, kaboom, that is all you need to fail. The platform is getting harder to develop for, as early entrants used techniques which were what SEOs might call "a wee bit grey" to gain traction, and these are either impossible now or will result in a new entrant getting hit with the banhammer. (For example, early social games maximized their viral coefficient by adopting a spam-early spam-often policy to user notifications, which ticked off a lot of Facebook users, causing Facebook to take a number of countermeasures. Ask somebody who builds for it to describe the specifics, but there are now throttles and some sort of system where you earn the right to use the viral channels.)