The issue is that social media strategists tend not to be strategic or tactical; the large corporations will continue to bring in those higher level strategists as they know that there is a need for that type of skill set (and the people with it are more limited than you would think). The good social media strategist is someone that understands and knows public relations and marketing and can work with marketing and public relations teams, as well as customer service, advertising and, at some levels, business development and align all to one group mind think. A group mind think that has a business value and proposition that extends beyond "hey, he's a nice person" but understands that social media campaigns need to translate to real business value.
The perfect social media is a quarterback, driving a strategy that leads to REAL business value, not popularity chasing with limited to no value. That position - the internal strategist that aligns various business units - will continue to be around, but only necessary at big corporations. The small companies and start-ups have no need for those people now, and will begin to see that there's no need for them in the future.
The social media specialist job, though, is a short-term job. Or, well, it should be (outside large corporations) as these are skills that any public relations or marketing person of any experience should have. Social media is just another term for community outreach, online communities, online engagement and those are skills that have just been repackaged and made sexy by people to get a jump on the competition. It's not that the large corporations don't have people with those skills, but there is a need for the alignment across business segments, having a single voice (or at least thought process).
So my thought is that many of these social media jobs will disappear within the next few years, if not faster. The job details will be spread around various people at companies - PR, marketing, customer service, community managers - and be managed by a person in the marketing or advertising departments at the company. The same would eventually happen at large corporations, albeit a bit slower as the larger the organization, the slower the process.
And while many of these internal people are talented, and will transition back into PR or marketing, a good number of them never had the basic skills and remade themselves into whatever was hot. What happened to all the SEO gurus and shops back in the day? Looks like a lot of them remade themselves into social media gurus and strategy shops. Expect to see those that had no real skills in the beginning to see the writing on the wall and begin remaking themselves for the new thing.