I used Prefundia.com, and I would again (free advertising is free advertising), but I think it is still small with not as much traffic as some other places.
Here's mine. They could really use a search bar with categories, especially as they get bigger. I received approximately 1 follower per 147 views, but none of those translated to backers as near as I can tell. Not yet anyway, and my Kickstarter's been live for 1 week, the Prefundia for about 45 days.
It will probably be more useful once they become better known and get more traffic and a search bar. I've gotten the most success from making relationships with potential backers, and posting on facebook. The problem with Twitter (and I highly recommend using it even more frequently than facebook posts), is that people are tweeting all day long, so your tweet could quickly become lost and never even read by the majority of people. I get hundreds of tweets a day from people and it's a part time job reading them all, so you would need to tweet several times a day, and you don't want to post too often about your project on facebook especially, or you'll annoy people. So, I do a lot of non Kickstarter posts there too, but this is getting off-topic.