In the last few years social media has become about more than Facebook and Twitter.
Other sites have been created that deal with a specific topic. Their membership numbers are small in number, but the topics discussed often garner a lot of activity because the people on the site are passionate about the subject.
When I think about subject-matter expertise in a newsroom, I think of a beat writer/reporter. In the newsroom, they are the subject-matter experts for the beat that they are assigned to cover.
They have a first-hand knowledge of the topic and the issues at hand and they have relationships with the people who are impacted or make decisions about the topic.
In the world of social media and online communities, they would be ideal community managers.