Now that everyone's getting in on the community thing, it's certain that only the appealing will survive.
But which sites will be appealing enough to attract a crowd that comes back again and again?
Air Miles has now launched a community site. This seems like a potential winner, since some people are pretty passionate about points collecting.
Browsing the discussions, I learned that you can collect points simply by searching on Yahoo. Which is a fascinating concept all by itself... I presume this works for Air Miles because of data mining possibilities. Which would turn some people off, but not dedicated collectors.
The home page has a scrolling list of things people have purchased with miles. Darren from Ontario just got an 8MB iPod Touch for 3150 miles. These items are clickable, and take you to the purchase page.
A lot of thought has gone into this site, to make it engaging. I bet it creates a stronger commitment to collecting and using miles among community members.
It's a good example of why you need a few things in place if you want a vibrant community:
 Understand your audience. You are probably not aiming to serve the average here, look first at your raving fans.
For example, one of the tips shared on the site is to have an extra Air Miles card in the glove compartment of the car as a back-up in case you forget the one you always have with you. Now that is a fan. That is the person you want to attract and keep happy.
Air Miles says "Learn Tips and Tricks. Discuss Ideas. Share Memories." This is really well defined, and the site supports these promises.
 Add a number of ways to interact and engage users. If it's boring, no one will come back. In addition to the features noted, I saw instant polls, highlighted tips, and similar eye-candy here and there.
Getting a new community going is difficult. One of the problems is that members ask questions and no-one answers them. This is where a moderator can come in handy. For example, Monique is looking for a place to stay in New York near Rockefeller Plaza. She posted the question on March 19. No one has responded. But surely a moderator could have answered this question, and should have. Monique is probably already disappointed. And seeing this kind of dangling question may discourage others from posting.
So when you build your community site, have a real plan. Why would anyone want to spend time there when there are plenty of other places to go.