There are a handful of companies that help measure your social media influence. They work pretty well using Twitter and Facebook as the main gauges, then blogs, flickr, LinkedIn, etc. I don't think they are a complete measure, but it's a darned good start. If you were hiring someone who is a social media expert, for example, I would definitely look them up on Klout.com.
My Klout score is 35. That's today. It has been as high as 41 when I'm looking, it changes day to day. They call me an "explorer" - and have this to say about that level: You're exploring the ecosystem and making it work for you. Your level of activity and engagement shows that you "get it", we predict you'll be moving up. It's not as high as it could be, but I've looked up some experts that are lower. It's a matter of practicing what we preach. It's also not the only true indicator of a social media expert's worth, it's only numbers after all, not personal.
Peer Index is a little more business-like than Klout. They bring LinkedIn into the equation.
Empire Avenue brings a gaming twist to the topic. You can buy and sell stock in people, thereby raising or lowering their worth, which is based on a real-life score that includes more input locations than the other Klout or Peer Index. Looks fun to me, I've only just signed up and none of my friends are there. That will definitely affect its success as far as I'm concerned. If people don't join, it's not very social, is it? Still, great concept. Come buy a piece of me!