In courses I've taken as a student (or casual learner in a MOOC), community has been tough to solidify online and often depends on the number of people involved. Too small a student base leaves the online communications feeling sparse; too large a user base (like in many MOOCs) creates too low a signal-to-noise ratio, although that will depend on the student's available time to keep up on things.

Most online courses I've taken made a point of requiring "classroom"-visible participation to some degree. In a few, it was required to simply make an introductory post to a classroom forum. I've often found that while this helps, it's easy to post an introduction and then move on. It's great to make sure everyone's able to participate but isn't enough to build community.

Assignments where students are required to give constructive feedback re: each other's posts on the course content have gone a lot farther. They open the door to students thinking about each other as fellow learners rather than just talking into an online void.