My account has been showing for a while that 88,000 of my 100,000 units have been used; however, as far as I can tell I haven't collected any new units for several months. It was my impression that the counter reset each month. Can you please clarify, and perhaps suggest ways that I can clear space to collect new data (besides, of course, just deleting archives/projects)?
The counter does not reset each month. Your account can store up to 100,000 items and as you note you still have room in the account to collect or upload more.
The Twitter feeds expired. The maximum number of fetches before you need to manually reset a Twitter feed is 100. Therefore, if you collect once per day, you will gather data 100 times over 100 days. If you collect more frequently, for example every hour, then the feeds will run down much faster.