As everybody else said, facebook is the largest social networking site in many aspects so it doesn't matter which characteristic we're measuring it on, probably facebook will be largest. And most social sites are easy to use but I do fail to understand your goal.
Probably twitter would take the second place, and LinkedIn may be the third.
Yet, the people that posts in twitter are a different kind of public than those who like facebook. Many people that uses facebook don't like twitter but many twitter uses do use facebook. That's why you have to address each public differently.
I don't understand the second part of your question... does it mean that if facebook wasn't easy to use, you would sign up for the second option?