I have often wondered about this too. I’ve just done a short, very unscientific survey by feeding four random sets of three letters each, comprising components of people’s names, into the search box: ‘geo’, ‘ann’, ‘pat’, and ‘rob’, then dividing the results into ‘active’ and ‘inactive’. By ‘active’ I mean anyone who has ever done a forum post; by ‘inactive’ I mean anyone who hasn’t, even though some of these are recorded as having made searches from time to time.
These are the results:
‘geo’: active 15; inactive 25
‘ann’: active 15; inactive 35
‘pat’: active 17; inactive 15
‘rob’: active 1; inactive 49
Totalling: active 48; inactive 124.
Some of the inactive members joined very recently, others joined as far back as 2013. Some of the active members are very active up to the present; others may have made their last post in 2014-16. And I’ve no idea whether the imbalance of the ‘rob’ results is comparable to any others, or a complete fluke.
But if these results are in any way typical, at 38.7% they compare very favourably with @tatjana’s estimate of 1/4 of the membership being active, and would give a figure of over 2,000 active members (though not necessarily all at the same time) if applied to her overall figure of 5652. But it’s a completely random snapshot.