Many authors have tried one or more of the social networks and felt it was a waste of time.
Maybe it was the wrong network?:
Whichever you choose, you need to plan on spending a little time building a base to your following.. inviting friends and participating as much as you can to learn the system and make the initial connections.
Which is best?
Depends on your book/topic. However, I look at it like any marketing problem.. I go where the readers are.. test the promising ones and keep the ones that work.
One things is certain. Moving forward, you should have a presence on at least some of these networks