I am looking for a way to measure the number of active users on the system over time.
I am mostly interested about StackOverflow, but this may also apply to the other SE sites as well, as I find it really interesting to analyse how this can vary based on different events.
Now by active users, I do consider people logged-in and that do have at least one contribution in a week (added at least one question, answer, comment or vote).
For trending, it doesn't make sense to measure active users for less than a week, as most people do not "live" on the website.
For research purpose, I am going to ignore consumers (visitors), people that do visit but do not contribute ever. I know that their number if huge but they need to be studied separately.