Subscription sounds like a solution - but building and maintaining subscription bases is expensive. Also how would you build a base in the first instance? It's normally through advertising. I would expect that if FB placed a $4 charge each month - they'd see their base drop 70%+ over night. From a user perspective although people don't like ads, they accept them as the trade-off v paying. Obviously if they're too intrusive and the reward is weak - people simply won't use them anymore