I seem to remember DoctorQ posting something like that at some point. I think something like 80% of registered users were a Regular or lower.
Edit: here is what I meant, although it is out of date and made by GGJstudios...
Thanks for posting this link; I remember reading it when it was first posted, and thinking at the time (sad nerd that I am) that this sort of thing is actually very interesting.
I'd say its currently around this. I'm a good example, been a member since 2009 and am only a regular. I just recently started posting more frequently.
This is true. People come and go - stuff happens in their lives and they drop out for months at a time, and then choose to - or, are in a position to - re-engage with the forum. Not long after I joined MR, at very short notice, I was sent abroad for two years, and more or less ceased posting for a good while, as work commitments took precedence.
In any case, I think many join to get an answer to a specific question when they buy an Apple computer - I know I did - and some stay on for other reasons.
However, one of the advantages of a forum such as this, is that people come and go as they see fit, hence you can sometimes see bursts of online activity interspersed with long periods of silence.