I participate in as few online communities as possible. I would prefer members of my online communities to do the same. Are members of 12 online communities really interested in developing lifelong relationships in mine? Are they interested in finding a group of people and working towards a common purpose?
Lets call these people skimmers. They skim through communities looking for short-term benefits rather than building long term relationships. When you go for fast growth and mass promotion you're most likely to attract skimmers. They are the easiest to reach. If you have thousands of members and little activity you probably have too many skimmers.
This provokes the bigger problem, how do you attract divers?
I see two ways. First, target the 50% of internet population which has never joined an online community. Convert people interested in the topic to the internet, rather than internet users to your topic. Go outside of the internet and existing online communication channels to find them. They're easy to reach.
Second, convert a skimmer to a diver. Set a rule, members can participate in no more than 3 online communities. Celebrate members that make difficult choices between which communities they spend their time. Ask members what other communities are offering which yours doesn't, and adapt to it. Flip the problem, specifically ask for veterans of other online communities willing to use their experience to help make yours the top community for it's topic. Aim for the type of people that are bored of the skimmer approach and want to become divers.