Here's what I like about niche communities:
1. Less clutter. There are fewer people and less noise on small networks, making you more likely to be noticed and become an influential member of the community. Rifling through profiles on LinkedIn can be like searching for the proverbial needle in a virtual haystack, but finding like-minded people on a less convoluted network is fairly easy.
2. Better discussion. The conversation tends to be more meaningful. This could be for several reasons, but my theory is that if people have taken the time to seek out a site dedicated to a certain topic, it's because they're passionate about it and really seeking good discussion.
3. Opportunity to learn. You can gain a wealth of knowledge if you pay attention. With Twitter or Facebook, you're constantly battling information overload. Rather than spreading yourself too thin trying to learn a little bit about everything on big generic sites, you can use small networks to focus on immersing yourself in one or two topics and mastering them.
4. Personalization. They're typically more customizable, allowing you to be more uniquely "you." Design your own background, post blogs or music or art, speak more freely, and develop a voice. Don't forget that once you send something out into cyberspace, you can never get it back, but I would suggest using these sites a little more liberally than your professional networks. Be genuine and let your personality shine.
5. Thoughtful feedback. Niche sites are an excellent source of outside feedback. Your friends on Facebook are going to sugarcoat, and you want to be careful about throwing around your raw ideas on LinkedIn. While Twitter can be good for feedback, it's limited to 140 characters. Smaller networks serve as an ideal solution if you're looking for detailed, thoughtful responses.
There are thousands of communities out there for every hobby, career, culture and interest. One of my personal favorites is 20-Something Bloggers, a place for bloggers like myself to meet online and share ideas. I also recommend Snooth for learning about wine, GoodReads for connecting with other bookish types, and Imeem for sharing a love of music.
If, for some reason, you're not able to find the community you're looking for, start your own! Sites like Ning allow you to build your own highly interactive social networks, complete with forums, chat, events, etc. Then, not only will you have an awesome network of people, you'll have become an entrepreneur in the process.