Web 2.0 is a child who must learn from it's mistakes, several website are  everything but a simple DOS (Duplication of Services) from another SNS (Social Networkign Sites) with nothing really more "interresting" or more relevent ...

So why does they built another one ? The keyword is, IMO, reasearch and developpement, the networking effect whith billions of services in a so short time is really a new factor ...

 But in a long terme some services will merge into much bigger one or die , only the most popular and relevents one will survive. The really important factor in the future of SNS, is to secure networks (trust, relevence ..) , because nothing won't stay in this way as "everybody's my friend" attitude for a long time ...

Remember all search engines it used to be by the past ... in france only 3 really stay and only one is making 85% of the market ...