I'm planning on quitting Facebook soon, and am currently going through the preparations necessary. One of the major reasons is the way Facebook uses the default privacy settings to fudge people towards giving up more information than they really intended to. There's a great example of this fact in action at We know what you're doing..., a website that posts public statuses of Facebook users in four categories: Who wants to get fired?, Who's hungover?, Who's taking drugs?, and Who's got a new phone number?
In their about page, they point out:
These people probably wouldn't want this info publishing, would they? Probably not to be fair, but it was their choice, or lack of, with regards to their account privacy settings. People have lost their jobs in the past due to some of the posts they put on Facebook, so maybe this demonstrates why. Efforts have been made to remove any personal data from the results, such as the actual phone numbers, surnames, etc. The data is still easily accessible from the API, the filters have been put in place to protect the site from legal issues.
The idea comes from a great performance by Tom Scott, which I'm embedding below:
A lot of the people on these sites don't know that they've left their Facebook pages this open. And that's the problem -- it's not enough to protect people's privacy to say, "You need to look at the privacy settings." Facebook buries them, and they set all the defaults to "Share everything." As a result, people who are on Facebook not because they want to stay in touch with the cutting edge of social technology but because they want to talk to their friends (read: damn near all of them) are unlikely to protect themselves.