A recent article from the BBC reports that students are using Facebook despite the age limitation on holding an account. For students younger than 13, they lie about their age to be granted an account.
The article has a European bias, but advice for parents is good no matter where you live. France has the lowest adoption rate for underage users, and likewise, has the most parents who forbid access to social networking.
I think social networking is fine for students – as long as participation has supervision – and the network/location is age-appropriate. Social networks that so easily allow contact between adults and students is likely not the best recipe.