I was actually working on something to measure this today. I may have something for you tomorrow. Otherwise, it'll probably be next week.
Update: I've gotten all the servers (Mbin, Piefed, Lemmy) and the stats for each (users, comments, posts). I'll need to work to make that data into something useful.
Trivia: it takes about 6 minutes to request data from all ~300 servers.
Update 2: I found, I think, some interesting analyses (ex. retention efficiency, current vs. historical activity, etc.). Not sure how I'm going to fit 300 servers with 25-30 columns each in a post, though. Problem for next week.
Update 3: improved building the server list (now >500 servers) and found a couple more analyses that might be interesting. Working on the post script, finish tomorrow or Monday. Also, I've been grabbing the data daily so date/days could be another area of analysis.
Update 4: I have a plan for how to post the analysis, now just implementing it. May see the post tonight.