Dont use them as a rule,sometimes i use twitter for promoting a new site but thats as far as it goes,dont like the idea of them gleaming my personal information hence thats is why all my details are not 'exact' to confuse them.
I have become an avid fan of "Pinterest" where I get an opportunity to pin topics of my interest. It is moreover like a virtual notice board which one can update on daily basis.
I prefer to use Facebook to build social relationship. But I also prefer to use Twitter and Linked-In as professional relations. The reason is that Facebook is for fun and useful to communication for social activities which Linked-In and Twitter helps to increase the professional relationships to increase the value of person. But for the promotion bases I prefer to use all Social networking sites.
My preferred Social Networks are Facebook, Twitter & Linkedin. These are helpful to interact not only with family & friends but also useful for business activities & promotions.