I didn't really get what the big deal was about at first, but after reading through the Are Like Buttons Evil?, I'm getting it a bit more. I can forgive Facebook for a lot of their sins, but defaulting a privacy policy in which other "affiliate" (read: advertisers) to open is fairly evil. The article goes more into how this isn't "open" (ie: the "like" button only goes back to facebook, not to digg/reddit/etc), but for me I'm more concerned about the security implications for people having to deal with the settings they don't understand.
To turn this off, log into facebook and then:
5 Steps through pretty nonspecificly named menus, to get to an option that a) gives your details to advertisers and b) defaults to on... sorry Facebook, but that's pretty evil. Of course, they know if they default it to 'off' no one would ever turn it on, so they wouldn't make their next bazillion dollars from our personal information to other big companies.