You're probably not maximizing the effectiveness of Facebook/Twitter to promote your community.
Most people use social media channels to promote the latest content in the community. This is fine. But you can go further.
You can use social media platforms to discuss what’s happening in the community.
e.g. Scott just replied with a good point about using Widgets for Wotsits http://dfth.bit.ly”
There are three benefits here. First, this sort of tweets is more likely to get clicked through. People are most motivated by what other peopel are doing. Second, by mentioning peopel by name you show recognition which is likely to cause others to participate more to ensure their name gets mentioned. Finally, it provides you with an ongoing stream of content to use to develop the community.