I posted some comments/notes from this article here. (Nothing new for people who already read the article :))
May 26 - June 1 Compliance-without-pressure techniques for Online Communities
There were a bunch of neat papers related to learning at the 2012 CHI conference a couple of weeks ago. This one compares the effectiveness of two different techniques for engaging people to contribute to an online "peer production community."
Although the example community is focused on geographic content, it seems relevant to the question of motivating participation in any online community. i also think the notion of learning as a "peer production community" isn't far off the mark (ie, artifacts are produced as a result of people learning with one another online - content, discussions, insights, papers, software, etc).
Hope it will be interesting!
Mikhil Masli and Loren Terveen, Evaluating compliance-without-pressure techniques for increasing participation in online communities, Proceedings of the 2012 ACM annual conference on Human Factors in Computing Systems, 2012.
pdf file can be retrieved here.