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)

FYI, bebo is dead in Europe, its Facebook all the way. (I think I heard AOL announced they were shutting Bebo down anyway.)

There are two questions this brings up to me, in relation to your points.

1) At one point are people willing to give up on Facebook w/o an alternative in enough numbers to seriously hurt the site.

2) How big is the time window that FB has to fix the problems before a competitor automatically could become a serious threat simply because of how fed up people are?