I feel your pain. I was lucky enough to be around to enjoy the days when the ONLY place online where people were discussing serious topics (and non-serious ones!) was USENET (and maybe The Well). Google Groups just started to throw a web interface on USENET. Sadly, once big money entered the Web in the late 90's, every Portal popping up felt like they had to include discussions/message boards into their features. Yah…
I tend to think that the 10% remaining still represents more users than what USENET ever had. It's okay to not include everybody in a web community; I actually think it's a mistake to try to include everybody. Some people just don't fit (and would fit better somewhere else). Many times I read people saying "in order to attract more people you should do this and that", but I disagree: maybe you don't want to attract the people you wouldn't attract in the current state.