Mart View Point (MVP)

This graph is interesting.

Check out quantcast.com to find out the traffic to those sites.

Not only facebook has the highest users among the social network, but also users there spend much more time in the site.

Facebook has reach a tipping point of no return. It's very hard to beat this site. The only way is to grab as many NEW users as you can, or convince facebook users to open another account in your site.