Well it depends.
I'm registered on three web-based Forums, but I only post on one (AO) on a moderate/regular basis. The other two I post on sporadically (although one gets more attention than the other).
I read Alfabb.com on most days of the week, but I'm not a member and don't post.
My preferred forum format, however is Usenet. It's quick to read, it doesn't hog computing resources, it doesn't require registration, and if you read unmoderatated groups, it's a bit biting in tone.
Currently there are two groups I read on a daily basis, and another handful or so I skim, but I've been using Usenet since the '94, so my electronic footprints are in all sorts of places.
Oh, and I forgot LISTSERV.
I'm down to one very low traffic list, but in the past I've been on some very heavy, time-intensive lists -- usually more than one at a time.