Social media really isn't media in the way we typically use the word today in public relations and marketing. Social media are your friends, your colleagues and your customers. Instead of a newspaper where you can place an ad to reach an audience, social media pushes right into their lives. For all practical purposes, it is a direct tie to your friends and customers. Because of its intimacy people have a higher expectation of etiquette and relevancy. Just like you can't cold call a friend (or customer) and try to sell them something, you can't quickly dial-up social media and sell your products.
This requirement of relevancy makes it nearly impossible to approach social media from a short-term campaign perspective. Social media looks a lot more like a long-term partner perspective where each works to deliver something valuable to the other. You have to listen for a long time and then approach the customer when they are ready for your message. PR people familiar with this approach - its called relationship building.
I was impressed with a keynote given by Loic Le Meur at ad:tech San Francisco (slides) where he said that Seesmic responds to every single customer Tweet. Within seconds, hundreds of Tweets from audience members soared from the auditorium challenging Seesmic to respond to them. I'm sure the customer response team hates it when he speaks.
I caught Le Meur immediately following his session and interviewed him in this video.
He urged digital marketers to stop thinking in terms of campaigns and to work to get real fans, not legions of fake followers. You can buy followers with enough marketing, but you want to really read what people are saying about your brand so you can respond and meet their needs.
Le Meur commented that fans are attracted by relevant content, not press releases. This commitment to content is the Achilles heal of any social marketing effort. People that are practiced in producing that type of content are difficult to find in traditional marketing departments. I agree with Paul Dunay's comment "When your content runs out, so does your social media audience."
Disclosure: ad:tech is a client.