"Measure what matters" has a lot great points pertaining to types of measurements, audiences and various other resourceful tips and tricks to the PR and marketing worlds. My favorite part of Katie Paine's book was the part of levels of engagement. Basically, it discusses the multiple types of audiences a company can have through social media.
Level 1: The Lurker
This is the person who only "likes" your company or organization on Facebook. Their "likes" may be from a contest or some other direct benefit from liking or following your company.
Level 2: The Casual
Casual engagements are those where they subscribe to your blog, follow on Twitter or suggest your page to a friend on Facebook. More involved with your organization and someone to focus on to draw in nearer.
Level 3: The Active
The people who fall into this category are those who actively participate in blogs, Facebook and other online sources. They re-tweet your company's information and genuinely care about how the company is progressing.
Level 4: The Committed
Committed people are those who have full faith in your company. They also will openly and actively participate on social media sites and encourage and positively influence others who are interested in your organization. You already have this group accounted for, but an occasion, "I'm glad we still have you around," is always a nice gesture and sure way to keep your committed audience around.
Level 5: The Loyalist
Loyalists might as well work for your company. They have a drive to improve the thought process of others in respects to your company, in most cases they are first responders to a concerned customer and are with the company through good and bad. The most difficult thing about loyalist is they are difficult to get an accurate measure on, as there are minimal ways to keep tabs on how this group is feeling.
Due to all of the levels of engagement, it is difficult to accurately measure social media success based solely on Facebook "likes" or Twitter followers. Thankfully, there are other ways to measure your company's success such as analytics, surveys, and interaction rates between your "likers" and followers. For further suggestions, I recommend "Measure what matters," by Katie Paine.