While I probably fall more into the "quantity" versus "quality" category, here is my take...
For me, it was purely an issue of time and such... I was a member of several and just couldn't keep up... I stepped back from all but one, which I am a mod on. Life adjusting and calming down a bit has given me the time to come back and reestablish myself.
I tried the FB groups, but I just couldn't get into them - they just seemed a lot more impersonal, and there was a higher percentage of people showing off and all that.
Going back to the days of the old alt.smokers.cigars newsgroup and into the era of the forums, to me the relationships formed are a lot more solid. I still keep in contact with some of the old ASC guys, and have herfed with people from all over the country with the forum I mod on over the years. That would have never happened with me on the FB groups.
Every forum has its own "board culture", personalities, brand loyalties, bombing etiquette, etc... I think that a lot of people are testing the waters to see if they can find a spot they can vibe with (to use the teenager slang that I am over-saturated with daily lol)
I may have veered off the thread topic a touch, but those are the ramblings that popped into my head...