Debbie Morrison asks in Presence, part 2 of 3 about online communities. She notes that some students like lurking in online learning communities and MOOCs. She notes that social presence cannot be easily defined or designed by educators
Social presence is felt by learners, yet is created by the course design and participation of other learners, in contrast to instructor presence which is mainly driven by instructor behaviours and participation
She also provides some suggestions and resources for exploring further
One Change a Day 
Makes sense. Quite simply, humans are social beings, so introducing that aspect to online learning is going to optimise the participation.
Yes – this is an interesting overview, there is a lot of research done by Robert Kraut too which found similar results about interactions