I come around about once a month to see what's happened. Maybe I post, maybe I don't, but I think the FB explanation is the proper one. Why? Because there's hardly been anyone *new* coming here. Most folks I've seen in terms of new faces are on reddit these days. That includes /r/Libertarian, /r/Austrian_Economics, /r/MarketAnarchism, and etc. And we're not the only ones seeing the decline btw. Mises.org's own forums are pretty much dead aside from the occasional progressive troll that comes in to stir the pot. Forums, like news groups and gopher sites, are dead. Ours is an unscaling technology, and it's not sexy anymore even though it had the potential to allow for the greatest distribution of communication power in history. People like sexy, shiny stuff over complex roll-your-own. Get use to it.
Edit: Also Second life has been turning into a dead zone as well. Basically, the dream of a distributed Internet is dead save for those of us working on stuff like Freedombox and a few other pro-user technologies.