Journalists rely heavily on their contacts to tip them off about news stories. Those contacts take a lot of time to make, and are based on trust, trust that the contacts won't get into trouble.
With social networks such as LinkedIn and FaceBook, a journalist's contacts could be easily checked. What happens if I write a news story say about Apple, and Steve Jobs gets upset about leaked information and checks to see who among my FaceBook friends works at Apple? And fires them? Even if that contact wasn't the source of the leak.
It is going to be risky being friends with a journalist...at least on the public social networks. Will journalists become social pariahs?
- - - Please also see Steve Rubel: The PR Theater of Operations Will Flee Email for Social Networks