Sometimes, it's the ignorant friend who posts the data that you want to keep private for you. Even if you're not a Facebook user.
[Tagged] "Grant Lawson passed out on the floor from too much cocaine at last night's orgy!!LOLOL!!!"
This is the argument I raise in most discussions about Facebook. While the above is perfectly feasible everywhere on the Web (not to mention other services such as Usenet,) the main problem is that Facebook encourages such behavior. Facebook doesn't only make business on making its users give up their own personal information (which I cannot object), but also others', mostly without their consent.
Take the contact list import function as an example. It asks you for your user name and password on a third party web mail (probably with a hypocritical "We promise not to abuse it" disclaimer) to download your whole address book and spam your peers. Anybody using such a function a decade ago would have been declared an idiot, but today it seems to be perfectly normal and acceptable to do so.