I am a member of many different forums, road racing(not street racing), 2.3L Turbocharged Ford engines, trucking, car audio, a specific International diesel engine, ect,..
Some are very specific and some are pretty broad in scope. All have a close community of people who often meet in person. I even took a cross country trip in my car that was only possible because of one of those forums. I had a string of jobs that was made possible by the forums too. Whenever I'm looking up some new or interesting topic the first thing I try to find is a forum that covers that topic.