I can state one secret community that's been inactive since 2015.
TheTrumanPlan
Every 6 or so months a random redditor was picked and the whole subreddit would set up scenarios to involve the Truman.
One example was when they made a whole subreddit for a band one of them liked. Once they found out the Truman joined, the subreddit was immediately privated.
They then began messing with him by
1: Pretending the sub got sponsored by a Korean ship company
2: make the Truman think there was an song from the band that they all loved but didn't exist.
3: Fake an AMA with a band member from the band and then have them mention the fake song
4: Have that Korean shipping company post that the AMA was fake and state they are not sponsoring the subreddit anymore.
5: Not noticing that they were slowly fading in Jim Carrey's face into the sidebar image for weeks and then telling the subreddit he was getting a tattoo of it. They all freaked out when they got told he got the tattoo I think lol.
That was all just from the first Truman I think. There were 7 more after that.
Here are the full megathreads from back then over the first two people picked. https://imgur.com/a/oHmC1NW
Holy crap there are at least 12 different servers within 50 miles of me. Some have a few thousand subscribers lol.
Gotta love Houston. Thanks for linking that!