It is my humble opinion that you can’t have a social media expert. Social media is a personal action, done by an individual. It is essentially “the act of sharing”. It’s thousands of years old and nothing new. So the only expert there can be is YOU, as it’s you that’s sharing. It’s a bit like breathing, a little hard to out-source. Yes, you could strap on a tank and do scuba (social media agency), but only for short periods. At some point you need to own it.
Using a platform such as facebook or twitter for the purposes of sharing on the semantic web (see footnotes) – that’s an altogether another scenario, and yes, you can have “practitioners” in these fields and in the short term they might have their place. (NOTE: I said practitioners, not experts. The web is ever evolving so you can never be at the top of the tree as the top keeps moving. They may be experienced practitioners but that’s who they are).
But the question is not one of “social media” as the answer. The question is social media on the web as an answer and why – i.e. why are you doing it and why are you doing it on the web?