According to Kevan Lee (Content Crafter, Buffer), testing phase in a social media marketing is an important phase, which include "What to test" and "how to test." Some questions you should ask yourself while testing are:

• Which one thing you will be testing?
• What metric will be my best measurement of success?
• Do I have enough data to assume this result will be true and repeatable for everyone?

In this article, the author writes about some tips for improving social media marketing test. They are: testing one variable at a time, look at the right data, compare consistently and check for statistical significance. 

To know more about the test used for various social media marketing sites, read on: http://www.entrepreneur.com/article/245478