I have to say that I did not know about the inurl google search parameter.
However, this is indeed a very rough estimate because the parameter 100...100000 also captures the number that indicates how long ago a user account was created, so it would also count accounts that were created a while ago (over 100 days) but were inactive.
for a numerical range find, which is VERY useful for date searches. A couple years ago, I gave a lesson to a class of mostly senior citizens for how to use Google to do genealogy work. We covered the range find, the inurl: and a couple other obscure but useful search enhancers. Pretty interesting stuff (here's the ODP, if interested) http://www.zentu.net/fmt/searchpresentation.odp
However, this is indeed a very rough estimate because the parameter 100...100000 also captures the number that indicates how long ago a user account was created, so it would also count accounts that were created a while ago (over 100 days) but were inactive.