We have just started using a new tool called Quintly to track, benchmark and optimise clients social media performance. It is very good. One lovely metric that stands out is the Engagement ratio. The ability to use one engagement metric across all platforms is a joy.
The engagement ratio equals the total number of comments, likes and shares divided by the number of posts multiplied by 100% divided by the number of fans.
The engagement ratio allows you to track how interesting each posted video, picture and word story really is, benchmark versus your competitor set and over time plan more interesting stories.
(If you also used the Google URL builder to track website traffic back to your website (if that was an objective) you could also measure the commercial effectiveness)
Along with the quality of your audience segmentation, the engagement ratio tracks whether you are finding the right people and telling them the right stories.