It (= getting the data of all forum users) has been done a few months ago by @Kumirei. The data are available somewhere on the forum. Let me see if I can dig it up.
Edit: Here
We had 39831 forum accounts with 0 posts, 56140 with 3 posts or less, so 16309 users who posted between 1 and 3 times, out of 68009 recorded forum users at the time.
So people with 1~3 posts account for 24% of all forum users and 58% of all accounts to have ever posted.
I guess it’s possible to filter for people who never returned, but I don’t know enough excel for that.