I personally do not use Facebook for my studies; I like to keep Facebook for personal and soical expression and relaxation. . I do however use Linkedin to connect with people professionally and join professional and educational discussion and I have also found Twitter useful for connecting professionally and sharing resources. It is the greatest networking tool to date.
I have come across an educational networking site called Edmodo.com that appears interesting but also looks to be in its early stages of development and I need to explore this further.
I do not use Twitter on a personal level to tweet about myself but I do like the idea of following others and sharing information and retweeting. I do feel though that even though I joined Twitter about 3 months ago I have not got the hang of it yet and I need to try to engage with it more to make it work for me.