1) I've never touched MySpace.
2) I have my own Facebook account, but use that simply for personal use. I know of some teachers that have two accounts - one for personal, the other for professional networking.
3) Twitter seems to be the most profitable for me. I love the resources that are shared and the immediacy of it. I would recommend anyone looking into social networking that Twitter be the first stop - then branch into other forms; whether it be MySpace or Facebook, own blog etc.
I did a survey of adult educators... didn't have many reply, but those that did all had facebook pages, but were not really using them for the classroom. Many had accounts and just didn't know what to do with them. The majority of them had all social networking sites blocked at work.
I agree with others that Twitter can be the most professionally-oriented of the main social networks; I follow Classroom 2.0 and others on Twitter and find many good leads for the classroom. I recommend following Classroom 2.0 and Mashable to get started on Twitter; also search for areas of interest to you, and find people to follow based on content. That's what distinguishes Twitter in my opinion: it's easy to find people who are interested in the same content as you and follow them. without getting into their personal lives. I use Twitter to share the most useful educational links I know of. (@benbohmfalk)
I think it's good to keep FB as a social, rather than professional, network, and to not "friend" students on FB. MySpace seems to be the most kid-oriented and least professional of the 3.
I highly recommend Edmodo.com for classroom use. No privacy or security concerns and 100% designed for the classroom.
I have Myspace and Facebook accounts for personal use; mostly because I've had the former account since I was 15, and had to open the latter during college to keep in touch with family members overseas. I started out using Twitter this year to keep in contact with family and friends, but over the last six months it became exclusively for professional use as I began to network with other educators.