There are some interesting social media statistics out there...
I think that social networking might offer some benefits in the classroom, although given that teachers need to maintain a professional boundary with students, social networking tools are of more benefit to a teacher connecting with other teachers. Establishing an online presence can serve to connect teachers to a large professional network that operates 24/7 around the globe. Beginning teachers can seek advice from expert teachers, and there is a sense of not being so alone knowing that many other teachers are blogging about the idiosyncrasies, the particular demands of, and ways to develop teaching.
One of the problems I see in networking and social media sites, is that as 'new media' voices can be echoed and teachers can begin feeling as though everyone feels the same way, even though only a small minority of people are joined to that conversation. This bias is something that needs careful scrutiny - how many teachers at your school are actually connected to Facebook, LinkedIn or Twitter? Probably less than you might think.
Our school has a mix of staff on any of these social networking sites. A majority use Facebook, but not for teaching purposes. Some are on LinkedIn, as a form of online/digital CV, whilst very few are on Twitter.
I find Twitter a great network for teachers, particularly with conversations such as #edchat and #tmmelb being some useful ones close to home that I follow. The ability to have these ongoing conversations with other education professionals is something that I have found beneficial as a beginning teacher. I also particularly enjoy being able to find teachers' blogs and reading about their experiences and opinions.
Overall I think the benefit of social media in the classroom, at this stage, belongs to empowered teachers being able to connect and share ideas, support and stories.