Twitter is great because it's an easy way to communicate with a large group of people, have a large following. What other sites can I join that require low participation but offers me a podium to communicate with large groups of followers? Sites like Digg and SU are nice but they are much more two way because "friends" will send you sites to digg and stumble.
I agree with marcus. Set up a Facebook fan page. The fan page is much better than creating a Facebook Group, and provides quality backlinks to your website.