If you’ve got ten minutes to spare, it’s well worth checking out this post on Shel’s blog.
While it’s long, it’s a great summary of the debate about who should be responsible for social media in an organisational context.
I agree with Shel’s conclusion that, in the absence of a specialist social media team, the PR/communications function within an organisation is probably where responsibility for social media should lie. He also has a good summary of the social media functions that such a team should fulfil. As he says, “answer is simple in concept, complex in execution”.
The post also makes the point that:

Corporations don’t communicate. People do.

That’s something worth remembering for communicators, particularly in my experience in internal communications where there is a desire for messages to come from the corporate centre, rather than a leader in the organisation – inherently weakening the traction that messages have with the intended audience.