We choose a new topic every two weeks with the online discussion leading up to a video meeting (although the two excessive posters don't attend the meeting).  For our last three topics the online totals were:

125 posts from 10 people, including 77 from two people. 

107 posts from 9 people, including 72 from the same two people. 

128 posts from 8 people, including 72 from the same two people. 

Of course it varies by day, with no posts sometimes for days at a time, but often you can get 5-10 posts/day from the same two people for several days running. 

We used to have more posters (and fewer posts), but I think that many of them felt drowned out and just gave up.