How to measure social media can be a tricky thing. Chris Lake at eConsultancy has a good article about measuring it with this step by step guide.

Take a snapshot

Before you start the clock it is a good idea to benchmark where you're at…

1. Make a note of the obvious numbers (number of Facebook fans, Twitter followers, Digg links, Delicious bookmarks, and referrals from social media sites, plus existing website traffic).
2. Make a note of the less obvious benchmarks (such as SEO rankings and referrals, customer satisfaction scores and other business data).
3. Make a note of ROI benchmarks. How much are you paying to acquire customers via other marketing channels? How vast is that advertising budget, and how is it being split up? And what proportion is being directed into channels that you cannot accurately measure?

Enjoy.