There is a "traffic" statistic that tracks the number of visitor sessions per day, but from a description of how it is calculated it looks like users can be counted multiple times if they visit several times in a day. While better than simply counting the number of people that have registered, the traffic stat isn't good enough because a small group of super-active people can make the site's numbers look higher than they really are.
As an example of why the current metrics are insufficient, someone on the Japanese Language and UsageJapanese Language and Usage beta site started tracking the public Area 51 metrics and plotting them on a graph:
The number of users (yellow) has been increasing constantly, but the number of "avid users" (blue) has remained almost flat. This suggests that the new users aren't really sticking around. Most of them probably register an account, maybe ask a question or two, and then vanish. My anecdotal memory of question askers backs this up - I tend to see the same set of names pretty frequently, and only rarely notice a new name or two.
The traffic (red) on the JLU site has been slowly trending upward with several spikes that correspond to a flurry of activity. Here's the questions per day graph for comparison:
Despite occasional spikes in activity and questions, the active user line receives no corresponding bump. Combined with my own anecdotal evidence again, this suggests that our traffic spikes are from the existing avid users.
So, none of the public Area 51 statistics really measure how well a community is doing, except in the most general terms. You can try to infer it by checking the reputation leagues and voter lists at the end of each month, but even that method is imperfect.
An actual "active user" statistic publicly provided by the site would make it easier to accurately measure the health of a beta community over time. The currently available metrics can be misleading and overestimate the activity of the userbase.