Measuring the results of social media efforts when they are based on how much revenue is produced can be measured by the amount of clicks/hits/visits/etc lead to purchases. However, measuring attitudes and opinions are often a bit more challenging since they can only be based on the feedback your client receives.
The best way to measure its success and popularity though is to track the amount of active, average monthly users and what percentage are returning versus new visitors. Additionally, you can track how often others spread and share your client's social media pages since that influence can lead to increased usage as well.
There are many different ways to tailor the measurement of social media outlets to fit your client. However, despite what their objectives might be, it is always crucial to constantly monitor the various pages to minimize risk and keep track of how successful and efficient their social media is being.