This question is a puzzling one. I know many marketing professionals--eyes wide and tongues wagging with the enthusiasm of a energetic puppydog--anxious to jump on the social media bandwagon to reach new potential customers; the particularly technically savvy youthful types with expendable incomes (otherwise known as the "holy grail" of target markets). Yet I also see social media plans coming down the public relations pipeline. So what gives? Is it the marketing or PR department who is in charge of a company's social media strategy?
A recent IDC report on purchasing behaviors highlights what many of us in marketing already knew: traditional advertising is out, content is king. This means simply that people (aka the market) respond better to messages when these messages are communicated in the context of content, lets say in this case an article or an influential blogger's opinion (present company included? *blush*). Consumers are generally skeptical and can practically smell it when they are spoon fed marketing messages meant to influence their buying behavior.
Given this premise, it would seem that the new social media approach to written content, like reaching journalists or influencing how companies are written about by bloggers, would fall in the PR camp; the undisputed camp of written content creation. And getting bloggers to blog about you is similar to--and just as difficult as--getting the traditional media mavens to include your company/product/service in an article. In this case add the complexity of trying to influence the readers-at-large in hopes that they Digg or Stumble you, and you've got a tough job; a job perhaps best handled by your friendly neighborhood PR professionals.
But what about many of the other content-related social media "tools" (*cringe*)? Aren't Web sites like Flickr, YouTube, and Second Life all about content too? Well, yes. However there is a significant difference between the type of content featured by bloggers and journalists and that included on these other media sites: who posted it. These sites and others like them all feature content created and posted by the public-at-large. This is also known as "user generated content." [I realize the definition of "blogger" continues to morph and that more and more of the general public are beginning to blog. Sigh, another added complexity.]
It is in this space where the question of "who owns it" becomes a little murky. One might say it is the domain of PR to influence how the company/product/service is featured in any external content, written or otherwise. Yet I can also envision a legitimate corporate marketing campaign that asks viewers to create content--like photos, videos, or avatars--and post them to these sites as part of a contest. This gets the users and the public engaged with your campaign and--pay attention, because this is your ultimate goal here--commenting and forwarding the videos to virally spread your message for you. [For example, see Intel's "What Would You Do For A Duo?" contest video.]
Another component of social media that falls a little more squarely in the lap of the marketing department are the social networking sites like MySpace, Facebook, and LinkedIn. These sites rely on the formation of groups to get messages out. Also, the observed behaviors of friends in your network by other trusted friends can ignite an idea and help it spread like a brush fire. (For example, my friend joined the group "Confused About Social Media" and I want to join too because I trust her and am likewise befuddled by the big idea.) Certainly companies can post traditional ads on these sites--which is not a terrible idea, by the way--but the most valuable features of these tools, I argue, are the micro-communities that culminate around the interests of its members.
After reading over this post, I realize I have failed to firmly answer your question--well, the question I posed in the beginning. Perhaps that is because the corporate world itself continues to grapple with how to and who should develop a corporate social media strategy. I would hope that any company's marketing and PR teams would work together to develop a cohesive social media plan. And that best practices learned from various teams and business groups would trickle up and down the ladder so that others can learn from your favorite feats and your most frustrating flops.