I do not understand yet what social networks are really good for besides displaying useless data to information junkies. For business networks this is a bit different but regarding the private use I am more scared than everything else.
While 30 years ago people in Germany boycotted the census, they now voluntarily offer far more and pretty sensible data to companies who only have financial interests .
What happens to all that data is not in our hands at all.
And besides that, I see lots of data that can easily be abused in the hands of governments or companies that want to select groups of people for whatever reasons ...