In business there is a theory that says all markets will eventually become a market of 3 major players or a monopoly. The internet definitely follows this theory. There are 3 big search engines. Ebay is a virtual monopoly as is craigslist.
The question these days is Social networking going to be a monopoly or a pie split by 3 players? I suppose it comes down to trying to how you broadly you define social networking. Myspace is virtually unchallenged as is facebook in their respective markets. It would therefor seem the only way new players to succeed is to create a new market as social networking will be a collection of monopolies, although they may be a series of successive short lived ones.
Looking at the long list of this and that clone being reviewed on techcrunch.com it amazes me how many people forget this simple "theory". It is simply not possible for another calender App, RSS reader etc to exist when there are already 20 competitors only 1 or 3 will ever live even if they had a business model.