I'm mainly here by a large margin now. Years back I was super into Rotten Tomatoes Forums when they were super happening and cool. The people who administered it fucked it up something awful. It became next to impossible to find threads anymore. I had saved some of the stuff to my own data, but only a tiny amount. It's such a shame they went under. There's something going on there now but I don't check it out, it's probably shit. Problem seems to be that the movie industry wants to promote their new films and that's the political dagger that's undone the forums... which were FREE, of course! Just my take/guess here.
I was deep into a few newsgroups back in the day when they were great and happening, probably my favorite being alt.home.repair, but they got super fucked up by spam, and that was pretty much that. I still have posts from those days saved in my email/newsgroup client (I guess a lot of that info is still good today). We're talking 2000-2009 or so, it got messed up later.
There are forums I participate in in a minor way. There's one thread I try to keep up with on semi-legacy equipment I use, MyHD HDTV cards and issues at AVS Forums. They are super deep in audio/video there.
I participate some at forum.thinkpads.com because I have a few Thinkpad laptops.
I go to GolfWRX occasionally 'cause I swing the clubs, etc.
I go to Tek-Tips forums for info on my database program, world class experts are super helpful and friendly there.
There's a DIY forum I go to once in a while, homesteadingtoday.com. Strong place.
I might hit Windows Central 'cause I have a Windows Phone.
I'm registered at a ton of other places but those are some of the ones I've been to in recent times. I have a number of products that have forums, so when I have issues I hit them sometimes.