That is a really interesting question - there will always be less 'active' members than registered as some people just like to read the posts without ever posting; no problem in my opinion. And there will be people who signed up, read a few posts and probably have never logged in again. That's how it goes.
The fact remains that there are enough active members here who have the knowledge and experience to answer even quite detailed questions and address tricky pinball problems, as well as give very good examples of how to restore a machine. To me that is where the primary value of the site lies; flowing from that is the occasional chance to meet up with other members at meets and so on.
If you look at the stats the most revealing one to me is the surge in new members in May and June last year (and even July), which is directly attributable to AP's presence at the Pinball Expo. What would be interesting is to work out how many of those new members stuck with it and are still reading posts today.