The 13th Law of Marketing, is the law of sacrifice.
You have to give something up to get something better.
Communities need to embrace this law. They need to sacrifice the majority of potential members and focus on a narrowly defined audience with a unique and strong common interest.
It's tempting to target all your customers (or everyone else in your ecosystem). But this wont work, the interest wont be as strong. Try to create a global community for parents and you'll fail. Try to create a community for moms in downtown Boston and you have a better chance of success.
The more similar the audience feels to each other, the tougher boundary they crossed, the more they are to participate. There are no shortage of studies related specifically to online communities backing this up.