Social Networks tend to be regional, bebo for europe, myspace in the US, friendster & hi5 for South East Asia, Orkut for Brazil, etc. If you decouple your game enough from social networks you can make them work on all of them and perhaps add incentives to use the more profitable networks over others. (10 Facebook credits only gets you 5 farmville points, but buying direct on farmville gives you 5 farmville points for the same price as 5 facebook credits, etc)