Leo Odden has blogged on the topic - How community building boosts SEO - this in fact posses a very relevant question. Can community buidling really help with website SEO? Personally I am not sure but it kind makes sense that it just might. Social networking is all about trust and connecting with people with similar interests or engaging in discussions which you may or may not agree with.
So when building those connections be it on Facebook or Twitter incorporating authoritative information and key knowledge on topics could surely lead to better SEO for any organisation. The organisation will appear in more places and potentially generate more links and mentions across the web.
Communications and PR comes together yet again in perfect harmony. All it takes is to think creatively and when posting to social communities use your knowledge of the keywords etc that are relevant to you and your site.