My forum attendance fluctuates. I've joined many over the years and tend to hammer one or two, then move on, only to return infrequently, if at all.
Sometimes a post one forum leads me to another, whereby I join, make a couple of posts, but then don't return, e.g. Piston Heads. There are only a few I keep going back to:
I joined MSE in 2006 and go there a couple of times a month. It's also the first place I go when I have a money issue.
I joined AVForums in ???? and again, spent a bit of time there to start with, but now don't go there very often, only if I have a specific issue, e.g. researching a purchase, or I'm bored and can't get my fix from any of the other places.
I joined Tyresmoke.net (motoring) in 2005 when I bought an Audi, but over the last few years didn't enjoy going there. It got quiet. The same people were posting the same stuff and I didn't like their opinions on most things. I felt outnumbered and I've not been there in several months and I'm not tempted to return. (Plus I sold the Audi and now drive an MPV, so I don't think I fit their demographic.)
Some others - bikemagic, outdoormagic, singletrackworld
I joined OCUK in 2012, but it's only in the last few months I've found myself coming here every day. I think it's due to the wide spread of members, different ages, different backgrounds, etc. It makes for a more enjoyable stay.