Once you simplify your second option it becomes actually pretty easy: count the number of people who logged in sometime in the last 30 days and subtract from th

Once you simplify your second option it becomes actually pretty easy: count the number of people who logged in sometime in the last 30 days and subtract from that the number of people who were signing up during that period. It should give you some sense how people are using your app at least 30 days into it. That would be the most basic MAU (monthly active users). I guess it boils down to what your goal is. Stickiness (repeatable action) is something very important to understand what really drives interaction. So, going in with some hypothesis on what you want users to do and then figure out if that actually holds true is a vital part at all stages.