Also, don't trust the numbers, even if they're cited. The current value for Twitter (500 million) is cited as being from an article in 2011 that mentioned 200 million. The Twitter company page - https://about.twitter.com/company - says the current official monthly actives is 313 million.
On the other hand I had found more answers about topics in specific subreddits with less users than in StackOverflow/StackExchange sites.
Context: It was immensely popular when I just entered university, which was in 2006.
You need to add "... Mentioned on Wikipedia" to the submitted title.
[Source: I used to work on one of the sites on this list, and its article went through AfD several times and it barely made it each time. It was never the number of users that saved it. Also it shows how stale this list is, because it shouldn't still be on there.]
When Facebook came around, most people left (mostly kids) because of parents getting on the platform. There's a video of a lonely woman that had contact with friends through Hyves, and said "But how am i supposed to send my friends animated pictures now :("
Even if it's "registered users" I'm not sure if I can believe it.
Does anyone know more about the site or is an active member?
Says 390,000 unique visitors for January 2015, according to ComScore. For comparison, DeviantArt has roughly the same number of "registered users", but gets about 6,000,000 uniques/month.
I suppose "registered users" is close to useless as a measure, because some may be automated spam accounts, some forums clean up dead accounts / others don't, etc.