I see your point, but 90% is just an arbitrary number I am guessing you have made. But number is going to be up there, so 90% might be a good guess. I probably won't pay monthly for facebook if a day like that comes. Unless you provide a service that provides constantly changing information of sorts, it is hard to charge customers monthly, I would have a hard time paying for the game scramble that I play on facebook. I might be tempted to pay them $5 once after trying the game for a month,

I might be wrong, but this might be worth exploring as a revenue stream for simple applications. Donations probably still do better than this.