Who's hired a SMM (Social media marketing) expert before? Reviews?

I was talking to a friend today who recently invested in a nightclub near Santana Row. We talked about various things, and hiring a social media marketing expert was one of them. He claims that one of the main reasons why his nightclub was successful, that it wouldn't have worked without someone who had all the right connections to spread news about his business and promote to people over social media platforms.

Now my question is this: Have you ever hired a SMM to market your tech startup? I realize that incubators like YC automatically generate a lot of free traffic for your startup, so I'm more interested in startups that are just starting out with not many connections. How was your experience with hiring a SMM, if you did?