Moves per day sounds like a pretty good and relatively simple stat to add to a user's profile page. For example, I signed up just short of 4 weeks ago (so 26 days). I have made 562 moves at the time of this post. Both these figures are easy to find. I have thus made an average of almost 22 moves per day. Since I have tended to be playing 8-10 games at a time, that is roughly 2 moves per day per game I have been playing (which is another stat that you might want to consider). (That is, I just mean to reinforce Gatecrasher's suggestions, not to say that this is different. It's just slightly more compicated to do moves per day per game, or maybe it's not. Depends on what figures are how easy to pull out of the database.)
On the other hand, one of my games has been going on for more than 3 weeks now since that player (provisional) has reduced his play frequency in our game to one move every 4-6 days after losing his Queen.
Not that it really matters all that much, but a game like that could throw a monkey wrench into someone's stats.