I recently found that a Wikipedia-related presentation of mine in a sociological conference (ASA in SF - let me know if anybody else is going there) got shifted to a Second Life track (it was supposed to be about online communities in general, but mysterious are the ways of conference and panel organizers).
So I thought I should at least pay a token attention to SL, and I thought about a brief numerical comparison. It is possible to consider the number of total registered accounts (en-Wiki 2009: ~10 mil, other Wikimedia Foundation projects 2009: ~10 mil, Second Life 2008: ~15 mil) but I run into a problem trying to get a wiki-equivalent of "average/highest number of active editors" (SL in early 2008: ~40,000 - but that's just for people logged in, doesn't say anything about their activity - one can leave a SL client running in the background... just like one can be logged in to Wikipedia for weeks).
What about Wiki? After some thought and discussion and a useful tool that was designed I begun compiling some stats based on a number of unique editors listed at RC page. There is still not enough data to see a clear trend, but numbers seem roughly consistent at about 800 registered users / 400 ips editing en-Wikipedia per hour and 90/30 per 5 minute periods.
As for Alexa traffic rank, SL is at megere ~4000, compared to our Top 10, but that of course simply reflects the fact that Wikipedia is useful for non-logged in users as well.
I wonder if we can indeed answer whether Wikipedia or SL is more popular? Wikipedia is more popular in general, but for logged in (registered) users, which one would be?