Social networks need to be interoperable. If it's not an open platform that anybody (meaning a new social network website) can join, then one site will gather all the users as other sites die off, and that site will become impossible to budge from its monopolistic position. To keep any one site from becoming too powerful, we need them all to operate on one free, open source protocol.
To an extent, however, you can fake that if you have good enough desktop software that presents one interface across all social network sites. It would need specialised plugins for each service, and they might be difficult to write if the sites are deliberately uncooperative, but it could be done. After that, the barrier to entry for new sites is much lower, since the desktop software can work with any of them, and you don't need to stay exclusively with one just because your friends are still there.
Yes, it would be complicated behind the scenes, but doing complicated things while presenting an easy user interface is what computers are for.
Mokalus of Borg
PS - I don't think this should be provided as a website.
PPS - But that's a different rant.