Active users is the standard metric used to check how much a service is used (at least as far as i know. its what i see when i look at stuff published for investors).
hexbar is on the sixth place in term of number of active users with 1.8K , lemmy.world is 18K (enable the "active users" column and sort by it to see the full list)
Similarweb can provide estimates, in October it it is 4.575B visits for x.com vs 75.87M for bsky.app . so about 1.63% of visits, so x.com isn't going away in time soon it seems.