What is the age limit for the various Social networking sites?

It depends on the system... anywhere from 13 to 18.

For Facebook, Bebo, Hi5, PerfSpot, and Netlog, the minimum is 13.

For My Space, the min. is 14.

For Friendster and Zopria, is it 16.

For Orkut and Yahoo! 360 it is 18.