82% of US teens use social networks for messaging

Pew Internet and American Life Project provided data on what US teenagers typically do on social networking sites.

Post messages to a friend’s page or wall 84%
Send private messages to a friend within the social networking system 82%
Post comments to a friend’s blog 76%
Send a bulletin or group message to all of your friends 61%
Wink, poke, give “e-props” or kudos to your friends 33%

Source: Pew Internet and American Life Report