It breeds narcissism. It makes ordinary people think they are special because they have 1000 virtual friends who would never pull their car out of the ditch in the middle of the night if you asked. These are not true friends, rather they are a symbol of false popularity.
Also, social networking is really just data mining and people willing give their information away for free.
And they do not bring people closer together, before social networking you would go see someone if you wanted to 'keep in touch'. It was more personal, more intimate. You loose a lot of body language, mannerisms, hand gestures, tone of voice etc.