If you hover over your name you will see your user id (at the end of a url) in the status bar of the browser. At least one other message board uses this number as a status symbol, a bit like many here do with post count and join date. The larger your id the newer (and therefore lowlier) you are.
Has it always been possible to find this number out in older versions of vbulletin?
I just answered my own question, because User 10K obviously knew his id. (10000)
With vBulletin it has always been possible; not so with UBB, the software we ran prior to vB. As this thread shows, everything has been discussed before at the SDMB 
Is it possible to learn what your user number will be before you actually register?
I’m just wondering how User 10K did that.
He could have simply noticed that the last user number was 9999, and knew when he signed up that he would get 10k.
Or, he asked to have it changed after the fact.
No, that was his original name.
At the time he was signing up, there was a “contest” thread speculating what the 10,000th poster would get, a teeshirt or mug…
Member 10K seems to have been aiming at the same target, but missed by 20.
Probably because the # of members on the main pages doesn’t count deleted members, like the mysterious member #1 (presumed to be an installation test name) #10,000
Member 10K seems to have faded after 12 posts, but User 10K was still around a year later to note this milestone: