I'll praised you that you gave an effort on creating a data analysis via time series but I don't it is the right way to use it. Time series is usually being use in the prices of assets to determine their movements through time. If you want to determine if their is some kind of seasonability when it comes to new members registering in the forum you should have put BTC's prices as an additional data to determine if there is some kind of connection why there are peaks in registration every two years. Simply putting two data internal to the forum won't give you as much of a reason why those peaks are happening.