The first stat I'd believe to be useful is "active users", maybe based on "logged in at least once in the last 30 days", to have a hint how many people use at least the webpage.
A second interesting stat could be "... mylists", either based on a certain minimum (>100 files = "established mylists"), or on a certain last-added-time (added 1 file within last 30 days = "active mylists").
Ideas? Objections? Critics? Flames? Tomatoes?
