Is there a way to find out how many registered users logged in a given time period? I am working on KPIs for my site and there are a few numbers I want to track. One of them is how many registered users actually logged in every month. Is there a way to get this number?
You could count all User table rows in the database where the fields for "Last Access" and or "Last Login" are newer than one month old .  You'd need to compare those timestamps to a timestamp for the start of the current month. You can find that with something like this or maybe it's as simple as:

$this_month_start = strtotime("-1 month");