A common trope in comment threads across the Internet on articles about Facebook's recent, myriad, astounding privacy fuck-ups is "Just don't post anything on the Internet that you don't want your employer or grandma to see. LOL DUUUUHH." This isn't terrible advice, but it completely misses the point.
There's nothing on my Facebook profile that would be actually embarrassing or harmful if it became accessible by the public. But I keep my privacy settings as high as I can because I'm only interested in sharing that information with my friends. It wouldn't be *terrible* if someone I wasn't friends with saw it. I'm just not interested in sharing with marketeers or random Internet people. It's none of their business. And that's reason enough.
Take another security context. I'm not opposed to strip-searches or backscatter X-rays at the airport because I'm secretly hiding weapons or drugs. I'm opposed to them because my body is simply none of the TSA's goddamn business. I'm opposed to unwanted exposure for its own sake, not because I'm fearful for the consequences of whatever's exposed. And, again, that's reason enough to be opposed.
Additionally, in the words of Cory Doctorow, "In any other context, making public something previously promised to remain private [as Facebook has done] is called 'lying.'" Facebook has broken a promise made to its millions of users that they would empower them to control who saw their content. It's broken its own freaking list of Principles for site governance. Facebook has lost its users' trust; we have no faith in Mark Zuckerberg's integrity or that of the rest of the company. It deserves to die.