the facebook is the only social networking site left where you can spend your time. Others are filled with lots of promotion and advertising stuff.
Facebook and Twitter are definitely the most used social networks. I'm using also Pinterest and Tumblr as well, and starting to use Google+! They are all good for building a good marketing strategy.
My top three are Facebook, Google Places, and Pinterest for business use. Facebook is the one I use for personal use.
Without any doubt Facebook. Now-a-days I am having some friend from webmaster forums. They all are added with me through Gtalk or Skype. So I am having Social relation with people not only by FB but also through forums, Gtalk and Skype.
I prefer to use Facebook as social networking for my communication.I think that this is the choice of 99% internet user out of 100.but i must tell you that this my personal opinion.