Myst URU: Online was a weird MMORPG made by Cyan Worlds, which allowed users to socialize, interact, and solve puzzles together.
It really wasn’t a success, as the target demographic is limited to myst cult members like me, i.e., effectively nobody.
One very interesting group was the “Skydiver’s Guild”. There were other guilds for other interests, but the unique thing about the Skydiver’s Guild was that they did things like this:
They would find places in the worlds where you could jump out of the boundaries, and then invite Guild members and others to jump off all at the same time, sometimes forming patterns. I had the chance to “attend” one such event (in, like, 2008) but I can’t find the screenshot.