Here's the results to the survey conducted by metaverse blogger Ryan Schultz, which asked readers on his blog, New World Notes, and users across several virtual world/VR themed Facebook pages, to state which virtual worlds they have an account with. (But not necessarily regularly use.) With 1,676 responses, this is a pretty decent, if unscientific, sample.
Unsurprisingly for this audience (and the world's 15 year existence), Second Life remains the proverbial 800 lbs. gorilla. More notable is that the top ten is dominated by "sons of Second Life", i.e. worlds that are closely related to SL in one direct way or another -- Sansar being created by the same company, Linden Lab, High Fidelity created by Second Life's co-founder (which is also funding Philip Rosedale's new company), Sinespace* being lead developed by Adam Frisby, a former SL entrepreneur who's also one of the co-founders of OpenSim -- which is itself an open source version of Second Life, while Kitely and InWorldz are themselves OpenSim spinoffs. So in the survey's top ten, VRChat, AltspaceVR, and Rec Room are the only three with no direct relationship to Second Life.
In terms of current usage, Second Life still gets the most concurrency (in the mid five figures every day), followed by VRChat (high four figures daily), and OpenSim-related worlds (probably low four figures, combined).
More commentary from Ryan here.