Okay, so this is up there with bears being catholic and popes defecating in the woods, as far as surprises go, but it's still good to have some figures backing up the widely-held assumption.
Mobile research firm M:Metrics says 70% of 13-17 year-olds are "engaging in social networking or otherwise creating content" using their mobile phones. Their definition includes photo messaging, video messaging, instant messaging, chat, dating and user-generated content
That said, the fact that this data doesn't include what many people consider to be true social networking – MySpace, Bebo etc – leaves room for speculation. MySpace has launched a limited mobile service in the US, and is negotiating with mobile operators over here in Europe, but it's yet to be seen whether teens (or indeed anyone else) will heavily use these services on mobile, and if so, what they'll want to do with them.