On many open wikis, a lot more accounts are registered than are actually used. This can lead to a very large user count on Special:Statistics, which can be confusing or misleading to the casual visitor.
For instance, English Wikipedia currently reports 6,794,875 registered user accounts. While reasonably accurate for what it records, this number is *vastly* inflated compared to the number of *active* user accounts, yet it's the number likely to be found and cited as the "number of Wikipedia users" by bloggers or journalists or curious individuals.
It would be helpful to maintain a running count of "active users", and present this in a more prominent place.
A suggested criterion is edit count; Erik's wikistats apparently provide figures for >5 and >100 edits in a given month.
We can either use a similar running count, or a since-the-beginning total count (which would be easier to maintain, since there's no need to decrement the count if a user has become inactive).
(As recommended by David Gerard on wikitech-l: http://lists.wikimedia.org/pipermail/wikitech-l/2008-April/037225.html )