How Many People Are Using Elgg Based Sites?

Greetings all!

I am in the process of looking into an interesting way to help the entire elgg community to grow and get funding in too. However, I need to have some rough figure of how many users Elgg sites support in total. I suspect there is no way to calculate this since there is no tracking built in to elgg, but if anyone has any idea somehow I would greatly appreciate you letting me know.

The site 'builtwith' does maintain a list of sites that it has tracked that run elgg, but they want me to pay hundreds of pounds to access the full list. Does anyone here have a subscription to 'builtwith'? That might help.