Whether it's an online community, YouTube competition, Facebook group or even a blog, you need to have a core group of people already there, already engaged.
Chris Brogan thinks you need to plant seeds. Maybe start some conversations for people to get involved when they join. He's right (he always is), but you also need to focus on getting to double-digits before promoting it around the web. I prefer the party metaphor. Imagine inviting hundreds of people to turn up one at a time to a party. If nobody's there, they will leave. It don't matter how good the venue looks, or how tasty the snacks are.
That's why you should always, always, begin you're online marketing efforts by making as many individual interactions as possible. Try to get 10 to 20 people together, and tighten the group a little. Now you can encourage them to invite friends and eventually promoting your online effort. Wait as long as you can possibly afford.
It's easy to take this a step further. Find those 10 - 20 people first, and ask them what sort of community, competition or blog they would like. It's easier to find the first 10 - 20 people then.
No-one turns up to an empty party twice. If you launch with nothing, you're ruining your chances of engaging these people in the future. We only have one shot at getting this right.