There's actually a few forums that are hosted on free hosting that are quite active. The Lost Media Wiki forum uses Proboards, and Sonic 3 Angel Island Revisited modding forum is hosted on Forumotion. The platform doesn't really matter much IMO, I know there are people who won't join a forum if it's hosted on a free platform but you'll still get people who don't mind that. If you're wanting to make your own forum, I think free forum hosting is a good way to dip your toes in. From there you could eventually just move to self hosting, when you feel comfortable. If you do want to move your forum to self hosting, use jcink since they offer database backups of your forum.