instruction: Amount of users who's last activity was during that month.
response: SELECT Month = DATEADD(MONTH, DATEDIFF(MONTH, 0, Last_activity), 0), 'Lost members' = COUNT(activity.UserId) FROM (SELECT UserId, Last_activity = MAX(Date) FROM Badges INNER JOIN Users ON Badges.UserId = Users.Id GROUP BY UserId) AS activity GROUP BY DATEADD(MONTH, DATEDIFF(MONTH, 0, Last_activity), 0) ORDER BY Month