Many organizations are using social media, and most are struggling with how to measure the impact of the effort. Here is one solution from The Measurement Standard: Social Media Measurement Checklist
The checklist leads you through the process, step by step, covering the decisions you need to make and every thing you need to do. You can use it to plan your measurement program, and use it to keep track of your progress. Every social media program is a little different, so you will may need to adapt the checklist to your own situation.