VRLFG.net is the place to go, pulling data from Steam's API to track people playing VR-compatible games and software. This came out of a conversation I had with a VR developer over this post last week, pointing out how few people seem to be using multiplayer VR. As of this writing (according to VRLFG) there's less than 2500 players total across all VR games on Steam (out of some 8-12 million concurrent Steam users). AltSpaceVR, which is perhaps closest to Sansar and High Fidelity in the social VR space, currently shows just 14 total players (23 total over the last 24 hours). There's an estimated 750,000-1.5 million owners of Oculus or Vive headsets now on the market, so you'd except at least 1% of them would be using their devices on Steam. But actual usage is well below 1%.
But hey, VR support is coming to MacBooks and MacOS, so that should totally change soon, right?