Well, we don't have a much better way of tracking existing OpenACS sites, given that anyone can download the software and play with it internally, or put up a public web site without letting us know. That is, after all, an inherent property of allowing completely free redistribution. There has been thought of including some code in OpenACS 4.x that would allow for opt-in reporting, so that a running OpenACS site can report its existence and traffic level to OpenACS.org.
