I disagree. The best niche communities are good because they are niche, not because they're hard to find. It's the focus and passion of the members that makes them what they are, and the ultra-niche nature of those communities is what ensures that people who aren't actually interested will stay away.

If it's impossible to find outside of word of mouth with acquantainces, then the purpose of the internet has been defeated. Why have the ability to connect all of these people with a common interest if you're going to refuse to let serendipity happen?