Let's look at Facebook: It as a service cannot function if our populace uses multiple separate social networks. A user will not be inconvenienced with logging-in to multiple services in order to interact with different friends, especially if said user is a casual user of the internet. Naturally, as a matter of a genuine desire to have a single place to meet with all acquaintances online easily, users will gravitate towards one service according to societal and peer pressures. We know this as Facebook today. Facebook has what we call a natural monopoly since it sufficiently meets the desires of its users and the requirement that it be the main service.
If it sufficiently met the desires of its users, then there's no way four dudes who happened to hear a speech by Eben Moglen about Facebook's privacy issues could have raised over $200,000 to start a decentralized social network. (Diaspora)
Anyway, there is no technical requirement that a functional social network needs to be centralized-- that just happens to be the architecture Facebook-- nor is there a requirement that everyone use the same social network-- a federation of social networks which can communicate with each other would suffice.