Who's going to disrupt them? Someone who can monetize the masses better? People are going to flock to a social network with more ads? With more fees?
I've consistently thought that people underestimate the lock-in Facebook has. They have one of the strongest, if not the strongest, network effect in the history of mankind. They can be disrupted, but only when there's some sort of major leap in technology that wholly redefines what a social network is. Adding incremental this or add-on that won't get anyone anywhere.
They can be disrupted by adding layers on top. Consider a browser extension or mobile app or whatever that aggregates social, and that proxies interactions with Facebook for you when you try to send something to someone on Facebook, but that first tries to do an end run against Facebook.
E.g. consider the abundance of "SMS apps" that sends the message as an IM over their own network if both sender and recipient are users, but that will revert to SMS otherwise. Many of them sell themselves as cheaper, but also as having more features, such a more flexibility in what you can send, instant read confirmations etc.
Now, there's plenty of chances for lawsuits and extended wars of attrition trying to block it for someone trying to do that to facebook, but I think long term they're going to be a too tempting target.
This is a good point to go into. At this point Facebook essentially needs to monetize their userbase to an extraordinary degree. That's the debt they've incurred due to the pricing of their IPO. That will likely force them to annoy their users with more ads, etc.
However, consider an upstart with nothing to prove. If they can put together a facebook competitive technology stack at a lower cost they can actually monetize less than facebook, and steal users that way. The fact that they would potentially make, say, half or a tenth as much revenue per user is irrelevant, because it would be their revenue vs. facebook's revenue.
I've consistently thought that people underestimate the lock-in Facebook has. They have one of the strongest, if not the strongest, network effect in the history of mankind. They can be disrupted, but only when there's some sort of major leap in technology that wholly redefines what a social network is. Adding incremental this or add-on that won't get anyone anywhere.