What data do you need?
https://openacs.org/projects/openacs/download/ gives you information how many times a package has been downloaded. This number does not include all the sites that run out of CVS (which all sites do that I know of).
As for the number of sites, if you write a quick spider to scan the net for /SYSTEM/dbtest and see if it returns "success" then you stand a good chance of detecting the number of sites that run OpenACS.
Alternatively you can look at the number of sites running AOLserver and assume that 70% of them run OpenACS (not sure if this is a fair number though).
I've been thinking about providing a functionality to the OpenACS toolkit that will automatically monitor sites that run OpenACS and tell the user that the site is down. For this, upon installation, the user has the option to say "monitor my site" and this information (aka URL, E-Mail of maintainer, maybe number of users, others...) will be send to openacs.org (or another central monitoring site), which will gather that data on a regular basis. Maybe, if we match the maintainers e-mail address with the openacs.org user database, we could extend the "about" page of a user to include all the sites he is running as a maintainer.
