Both, Area 51 and stackexchange.com display the total number of users as one of the statistics available for each site. Area 51 additionally displays the number of avid users defined by meeting a certain threshold of reputation on the site. There is also a graph of new users in the moderator tools.
I consider the total number of users, as it is displayed currently, a completely useless statistic. I have no way to determine how many of those users are actually active on the site, I can only see how many people have registered. Many of those might never have come back to the site. The number of new users in the moderator tools is also not very useful, as I have no idea if those new users actually come back to the site and contribute to it.
The number of avid users on Area 51 is also not a sufficient statistic, as it doesn't tell us if those high reputation users are still contributing to the site. Most users don't rage-quit and have their account deleted, they just fade away silently. Most sites likely have some users with a lot of reputation that are not contributing anymore.
The most important thing I want to see from the user statistics is whether the site is gaining or losing active users. A site that is steadily gaining users that actively participate in the site, either by contributing posts or maybe just by reading and voting on the content, is a site I would consider healthy (barring any other severe problems). If a site that is hemorrhaging users, if the number of active users is shrinking over time that would be a very clear warning sign that there are severe problems with the site.
If the user base of a site is shrinking, it would be useful to see this problem as soon as possible to be able to take countermeasures. This is where a better statistic for the number of active users would be very handy. This would be especially important on beta sites, to better gauge the health of the site.
For this statistic we need to define what constitutes an "active user", some ideas would be
- Has visited the site in the last 30 days
- Has performed at least x actions in the last 30 days (actions would be new posts or edits, maybe also voting and commenting)
- Was active at least once before (to filter out new users, this statistic should show the people who stick around)
I've create a very basic SEDE query that shows the number of active users (posted at least once in the last 30 days) and here is another query from Sklivvz. They give very different results, probably some error in my query. Though more interesting would be the time-dependence of that number, but my SQL knowledge is not sufficient for that query.
One might also split the statistic into active users and passive users, who only read the site, but do so regularly.
This statistic would be most interesting for beta sites, so a SEDE query would be not possible. Integration with the moderator tools, so that the mods could see the number of active users over time would be most useful.