It all depends how you define 'active'.
I conducted a month-long survey on this same theme, and estimated that 1 in 5 of IUOMA members was 'active'.
There are now (31/03/2015) 3874 members, which means that about 7-800 of them are active.
(See my Blog, "BIRTHDAYS TODAY AND EVERYDAY 22 DECEMBER, 2013 TO 21 JANUARY 2014 . IUOMA's 'REAL' MEMBERSHIP" of 28/12/2013 for the analysis)
I drew the following conclusions):
i) about 20% of IUOMA's Members are 'Active' as I have defined it.
ii) the remaining 80% may or may not be active Mail Artists outside the IUOMA network
iii) at one time this 80% were active IUOMA Mail Artists --that's why they joined (but see vii) below!)
iv) it is not necessary to be in any network to be a Mail Artist (well, I suppose you have to have at least one other person in your network to send Mail Art too!)
v) within IUOMA, a small number of people are responsible for a large amount of activity. They are the 'Super Actives'.
vi) as to the rest of us, well, we have different levels of Mail Art activity. I see it a bit like a love affair: start enthusiastically and with great passion, but then what you do and how vigorously you do it sort of tails off over time, and eventually you just get to live with your Mail Art personage.
vii) many IUOMA Mail Artists have been and gone, and, I trust, are still Mail Arting. As Mail Art is supposed to be enjoyable, and it certainly is for me, I would like to dedicate this whole Blog to one of our number who you have probably never heard of*, but whose approach to Mail Art somehow seems to encapsulate what it is all about: fun
*Deleteme", whose Profile Information reads;
Why I am involved in Mail-Art:
My Mail-Adress (snail and/or E-mail)
She became a member of IUOMA, joined one Group, but never posted a message, photo, or blog, or joined a discussion, and has not been heard of or from since.