I'd say the other big advantage - and to my mind even more important than the social aspect - is the access to the tutors and professors. Having done a number of online courses, they're fine as far as they go, but even a well run forum is no substitute for actually being able to talk to the subject expert, get him/her to walk you through the problems...

Edit:

There's another major advantage: formal qualification / certification. There's obviously nothing to stop me cheating at an online course, the certificate or congratulatory email they give you at the end is nearly worthless.