Much of privacy law is based off what society considers to be a reasonable expectation of privacy, and our behavior on Facebook, Twitter, and so on is setting the bar damned low.
There was a terrific essay on this subject by Alex Kozinski, Chief Judge of the US Court of Appeals for the Ninth Circuit: http://www.stanfordlawreview.org/online/privacy-paradox/dead...
The last couple of sentences in that essay sum up the problem nicely:
If we the people don’t consider our own privacy terribly valuable, we cannot count on government—with its many legitimate worries about law-breaking and security—to guard it for us.
Which is to say that the concerns that have been raised about the erosion of our right to privacy are, indeed, legitimate, but misdirected. The danger here is not Big Brother; the government, and especially Congress, have been commendably restrained, all things considered. The danger comes from a different source altogether. In the immortal words of Pogo: “We have met the enemy and he is us.”
If the police are investigating X, searching his house requires a warrant. However, going to his grocer and interrogating said grocer and getting records from said grocer has been less protected in the past. The grocer isn't the target of the investigation and so has less defense against being required to divulge information about the target.
Problem now is that everything we do online has these intermediaries - other entities who hold lots of data about us - involved, and so "attacking the intermediary" has been exposed as a major privacy loophole.