I see three questions within your post. I'll attempt to address all three.
1. How can we get new members?
Promote the site. Reinstituted the "invite" feature, setup adwords, get as many BLs as possible, submit articles, the standard SEO martketing techniques.
2. How can we get 100% of registered members to be active members?
Only one way, really, and that's even debatable. To have a 100% active membership, if a member is inactive, cancel his/her membership. I don't like doing that. Other than that, achieving 100% activity is not going to realistically happen. That's like trying to get 100% of registered voters to actually vote. It's a nice ideal, but not realistic.
3. How do we generate more quality content?
This is dependent on a few factors. First, changes in the SEO world will generate new questions, concerns, and ideas. Otherwise, if the same group has already taught and learned all they can, then no growth occurs. Second, new members, whether they be less or more experienced, will ask new questions and propose new ideas. So new members are needed. Third, since I'm sure nobody in here knows absolutely everything SEO, there is always room to learn something. Once that new knowledge is obtained, (s)he should inform us, which will generate new quality content. Fourth, we start posting more and spend less time in the general areas. The problem is the general areas are part of the life of this place.
God knows where you are, but Google may not. SEO is the key that unlocks search engine success (and a plugin isn't enough).