Okay, so I haven't exactly scoured the entire web looking for all possible MI-related forums... but I've been over at the Mojo and MI forums at LucasForums.com (since being told about LordTrilobite's SMI:SE fixes,) and they feel about as alive as a pewter wad after you put grog in a mug and wait too long.
By contrast, the Telltale forums are packed with people who post all the time. Probably says something about the complete awesomeness that is Telltale Games, but I still feel compelled to wonder how this place has managed to become the most active Monkey Island forums on the whole interwebs.
I mean, MI has been around for 20 years, and it's quite possibly the most popular adventure game franchise. You'd think there'd be more activity elsewhere, not just right here.