I run an internet forum for a very specific topic. I have to manually register people, because before I did that, spammers would come in and crap all over everything. (Fortunately it's not a very popular topic, so I only have to register new accounts a few times each month.) I run the forum on my own dime, no advertising or anything, as a side hobby.

There's also a very active Facebook group. The Facebook group is great for general conversation, but often when a technical question comes up, please just link to the forum where the info is stored. Searching in Facebook is terrible, and what happens if Facebook decides to block access to history for some reason? (Not that they necessarily would, but I've seen it happen many times. Remember when Photobucket blocked access to old pictures unless you had a paid account? We lost a bunch of useful pictures on the forum when that happened.)