It's a true federated system; you can run your own server, you can have portals to other servers, and, for grids which sell things, there are competing payment rails.
Open Simulator has a lot of spaces, but not that many users. Like most federated systems, it's a niche. Works OK, hard to use, few are interested.
Second Life continues to plug along. Right now, 50,844 users are in world. Which is more than any non-game metaverse. (Not sure about Meta; they don't give out numbers, but 20,000 has been mentioned.)
MMO games are far bigger; check Steamcharts. The top games are in the millions.
Don't believe any number about a virtual world you can't check from the outside. Concurrent users right now is usually the only honest number. There are systems which claim huge numbers of users, but their definition of "user" is "they, or some bot, put an email into the signup form."
The "metaverse" hype has produced a bit of growth, as people find out that most of the hyped systems are either nonexistent or very low rez. Usage is very low. Decentraland is around 1000-2000 concurrent users. They got up to 2600 once. Cryptovoxels is smaller. So is Sandbox. Sominium Space is in single digits.
that list is horribly outdated - so many of the grids in the list dont exist
Most people these days run their own grid through the dreamgrid installer. that actually increases fragmentation
and then the testing grid , osgrid, is down for multiple days every few weeks, which alienates and frustrates new users . overall activity is down, despite the pandemic. and my experience is that it's very very hard to keep users interested because it is lacking the critical mass of people. It also suffers from very high levels of drama
i find it very hard to believe that decentraland will keep such high numbers for long, it has received enormous coverage in the media eventhough it's bordeline a scam (imho). opensimulator has not received any of this
I dont think metaverses should be compared with mmo games - they are different things and attract nonoverlapping crowds.
But even the "free version" of SL has minimal traffic: https://opensimworld.com/