Fascinating stuff, and fascinating, as always, to read.
My sense is that the choice to engage in some degree of active participation comes and goes, and is often mirrored in what is happening in someone's real life.
When I joined, I went through a very prolific period for my first six months, but mid way through that time, I was posted abroad. The end of my first active posting period more or less coincided with a move to a much more senior position, which meant that time became much more limited. Moreover, my interest waned, as well, to a certain extent.
However, as Apple fanboy has already pointed out, sometimes MR provides an outlet which allows for sanity; other times, it is a factor in generating insanity (certain threads on certain forums, all of which shall remain nameless come to mind). And yes, it may allow for some reflections concerning one's social life……or lack thereof….