To those who say, "Nothing to fear if you have nothing to hide..."
Sometimes, people are using the internet for more than porn and kitten pictures (shocking, I know). While there are sites like LinkdIn to use for "business" contacts, it's not nearly on the level that Facebook is. However, that doesn't mean you want to or should be mixing business and pleasure. Additionally, some people do try to keep their personal lives separate from the public and/or employment piece, if only to prevent someone using things unrelated to work against them (see the Tiger Woods saga... err... wait, you can't talk about that here... see the growing list of employers who use interns and such to troll for personal details like affiliations and memberships on the internet when considering you for employment).
Idiots who post pictures of the keg stand from last weekend and make it available to everyone have it coming when a spouse or significant other sees them also making out with the cute blonde who isn't them. A teacher who does not want to or is professionally restricted from being "casual" with students is entirely another. Should a teacher than completely avoid any form of online networking and become the 21st century equivalent of a hermit, on the off chance a student might find them? Or should sites like Facebook comply with common sense, user requests and the law to give users the control over who can see what on their profile? It's not like whatever you post on the site is prevented from being used in data mining operations if you set it to "Friends Only." Facebook can pretend to make money on it's dataset with the user in it.
It seems like little to ask, just like it's little to ask the old phone book companies to keep your name and address unlisted, or asking to be on the Do Not Call list to cut down solicitations. Not only that, but there is something to be said of the dystopian ideal that all information is free and freely given on the internet. But if it is, when some script kiddy steals your credit card info from a secure site that turns out to be anything but, are you saying, "Fair game, kid. Go spend to your heart's content?" No, you're calling the cops to nail both the little schmuck and the company you did business with to the wall, and then send them away to experience the tenderness of some dude named Butch in a prison shower.
Plus, do you really want to parrot the likes of Bush and Cheney and other American right-wing idiots by saying, "Let us pry into everything about you. If you're innocent, you should agree that our efforts to catch the 1 out of 50,000,000 who are a threat to you are completely in line." Granted, the UK is now known far and wide as the most frightening example of a self-inflicted police state. Let's hope everyone else can learn from that.