Back in the day (freshman @ uni so 2006) all the cool kids were making facebooks. You had to have a .edu email address which let you into that uni's network. Only the students/recently graduated could participate. Now everyone and their dog have facebook accounts.
To make something popular and or "exclusive" one needs to have a niche of exclusive/cool people willing to use that product and a bunch of other people that don't have access to it. This is EXACTALLY what Zuck did at Harvard. He created the ultimate cool kids club at the most exclusive university in the world. This would have been impossible at almost any other university, also Harvard doesn't have the nerdy reputation of some other universities in Massachusetts. Then they opened it up to the masses, and all the parents/people who are not students/hs students got accounts and the exclusivity went downhill. But that isn't their main strategy anymore, it is to keep people yearning for human interaction, but without any negative social consequences... There is a VERY good reason that it is not a good idea to ask a girl/guy on a date via facebook, which is the exact same reason which makes it so popular aka no negative social consequences (can look @ the cute girls bikini pictures all day long and you don't get yelled at) but only to an extent. That is the facecrack.
So yes, they will be able to maintain the facecrack of pseudo-social interactions. They will not be able to grow with the exclusivity that they once had though. This is until the next exclusive thing comes along though.
tldr:
1. FB grew out of Harvard and associated exclusivity & worldwide appeal
2. FB uses pseudo-social interactions to give its users a limited amount of good feelings, but does not have the consequences of real life interactions.
3. FB will cease to be king of the mud pit if something more exclusive comes along.