Andrew K Kirk, Founder of Face The Buzz and Social Media Examiner writer, outlines 4 Social Media Goals Every Business Should Measure, but these can all easily be applied to libraries as well.
"Goal #1: Raise Awareness of Your Brand
Goal #2: Website Traffic
Goal #3: Website Visitor Loyalty
Goal #4: Conversion Rates"
Via Judy O'Connell, Karen du Toit, Michael Stephens