I decided to do a little math. In this case, I'll assume that we are at 100,000 posts and there are 200 members. This means that the average number of posts any member on GOF makes is 500.
Now I'll compare that to another forum I'm familiar with, bzpower (the bionicle fansite). They recently hit 5,000,000 posts, and there are nearly 40,000 members. This means that the average number of posts for any member there is only 125.
Considering this, I believe that what makes GOF so great is not the number of members, but the activity of those members. We probably have a large percentage of members here who join and become active rather than join, stay for a bit, and then leave.