I have been at so many internet marketing events where every other person is a social media consultant. So I ask, "If you hired a social media consultant, what factors or qualities are you looking for?"
Someone who can understand my product or service and be creative enough to come up with unique ways to spread buzz about it and make it viral ... and then actually execute those ideas with positive results. Or at least (in the hiring process), see an existing portfolio where such was done for other companies with similar business models.
Dani nailed it on the head. As for how many social media consultants have a portfolio, I doubt very many and of the ones that do I would wonder how many of the samples they would show you are actual work they did for real companies or if they created samples to fill their portfolio. As sad as that sounds, it does happen. I used to deal with a similar situation with so-called PR consultants back in the 90's. They would show me samples with company names and dates redacted ' to preserve the privacy agreement they signed with their other clients.' Then when I told them that I would need some references from some of the companies that the samples come from and they start to stammer and stall and I knew they were full of it.