Simon's spot-on here as to why Google/Facebook/etc. would be a bad move. The first impulse of most projects is to pull in social, so being mindful of how a user is asked to approach your app is a great place to start.

Cole Stryker has written a couple neat books on the (often constructive) power of anonymous social networks — http://www.amazon.com/dp/1590209745 — although the chasm between Anonymous and a teen support network may be too far to bridge.