Beat Saber is possibly more popular than VRchat, when desktoppers are excluded:

>VRchat = 11,519
According to Steam charts, this includes desktoppers and SteamVR users, but not Oculus users not using SteamVR.

According to Steam's VR section and Steam charts: 
>#1 = Beat Saber = 1,960
>#2 = VRchat
>#3 = Alyx = 1,480

So VRchat has fewer than 1960 using SteamVR right now, but more than 1480. How many are running through Oculus alone is unknown. I doubt that number exceeds the SteamVR players. VRchat probably has fewer than 3000 VR users in total right now.