French social network?

If Japan has Mixi, Spain has Tuienti and obviously most in America are using Facebook or Twitter what is the main social sites French people use? I know some French people also use FB and Twitter but is there no local network dedicated to French?

edit Also can I register for these since I am an American? For example Mixi requires a Japanese cell phone for activation (you have to live in Japan to have one) and Tuiteni is invite only.