How to better use social networks

It is debatable issue that use of social networks like face book, orkut, twitter, linkedin etc is beneficial or it is just waste of time and mind. I am using face book, twitter and linkedIn and found these useful for promoting my profile, publications and also to be in touch with many of my friends. While using any technology product you must be choosy, else you are going to be indulge in non related features and hence proving the product non beneficial.
So it is required to be selective and have some goals in mind while using social networks. Goals may be some or many depending upon your interests.