First, there needs to be a critical mass of interest before people will want to sign up. There's no value added if you go to a site and find you're the only one near you interested in the topic. So there are strong network effects: the more people are interested, the more likely you are to have a local match. Meetup.com attempts to be a central clearinghouse on a range of topics. But for most topics they seem to have at most a couple thousand people signed up worldwide. That means if you're not in a major city you're probably not going to find anyone near you. One technique that could help in this light is to have variable geographic boundaries. For some topics, and in some rural areas, people might be glad to be matched up with people even a few hundred miles away. As more people express an interest in some topic, you can be matched with people closer and closer to you (zip codes allow for pretty good distance calculations). Perhaps meetup.com will move in that direction. The Dean campaign is making use of meetup.com, but also has its own matching system, where they let the user set a maximum distance from their zipcode for events they'd be interested in. The system could probably suggest the distance threshold automatically, so that users would always get some search results.
Second, because of the need for critical mass, there has to be a focal point; most people need to guess the same web site to go. For some topics, such as a presidential campaign, there is an official site that is the natural focal point. For others, it's not so clear.There may be strong network effects here, yielding a winner-take-all market. If meetup or someone else (YahooGroups if it wanted to get in the game?) gains enough attention, it could become the place that everyone would think to go on any topic. Once that happens, it would be hard for anyone else to get in the game, much as it's hard for any auction site to compete with eBay at this point.