Of course, how often a software is actually installed is an interesting number for each project. So it is for rsyslog. And, of course, we do not have any data. While you meet some users on the forum and mailing list, they are only a very tiny subset of the user basis. With infrastructure projects like rsyslog, people often do not even know that they run it (what a shame...). Anyhow, it is motivational (and useful for promotions) to know how often it is installed.
So I started to search for some metrics. A good starting point is the observation that beginning with version 8, rsyslog is the default syslogd for Fedora. So, basically, each instance of Fedora 8+ means an instance of rsyslog. Thankfully, I was able to find some metrics directly from the Fedora project. If I sum up the metrics for F8 to F10, I have around 8 million systems. I guess this includes upgrades and now-dead systems. So we are probably down to 5 million (or is this too optimistic?). As a side-note, I agree that some folks may remove rsyslog in spite of some other logging system. But that will probably be special cases, so I don't think it is useful to try to hard to find out a decent number of these (aka "I ignore that" ;)).
The next major source of installations is probably Debian Lenny. Since a few month, rsyslog has become there default syslogd, too. I have not yet found any metrics for Lenny (do you know? - if so, please mail me). I think the number will be way lower than current Fedora (given that it is not yet flagged as stable). So it will probably not add a big number of systems, maybe half a million?
Another source may be several smaller distributions (like centos) where rsyslog is the default. This adds another source of installations.
Finally, we have the cases where folks intentionally install rsyslog. Sadly, these are the fewest cases, but as I said: this is what you expect from an infrastructure project. And logging is definitely a niche. Few folks have big interest in it. So, if looking just for numbers, these cases are almost irrelevant (of course, from any other aspect these are the most important ones for the project, they really drive it!).
Not having any real Debian metrics, I think a reasonable conclusion is that we have around 5 million systems running rsyslog by today (January 2009). I'll probably refer to that number if someone asks (and some folks begin to ask). If you have a different opinion, metrics, ideas - please comment to this post or email me.