I've wondered that myself. I figured one way or the other if you are serious about collecting pets and put any true effort into it you will end up having to use Google, eventually you will find yourself here. After I had about 400 or 500 pets and was seriously collecting them the wowhead page never seemed to really help or answer specific questions so I wound up here. So I would say there are quite a few collectors that dont use or know of the site but are probably nowhere near anything noteworthy.
It's akin to asking how many potential Olympic atheletes are there that never make it to the Olympics? Many I am sure, but the ones that pursue excellence and really strive to be the best at something, someone would have to notice them they would have to stick out at some point. I mean you would almost have to be an island unto yourself and have no contact outside of just general wow and even in game. I mean you eventually find other collectors and spark a conversation and pick up tips and tricks. I'm sure there is someone out there who could be top 500 or higher and doesn't use this site, but it would be highly unlikely.
Whats the point of amassing such an impressive collection if not to show it off?