The web has, since the very beginning I think, had the notion of public and private resources, where access to private resources requires authentication. I view facebook as an almost entirely private resource, protected by a user name, password, and terms of use.

So, they can do what they want within the law, so far as I care. I only get annoyed when people engage in hostile behaviour on the public internet - resources that we're meant to freely read, share links to, or index, or process in whatever way we choose.

As far as I can tell, facebook do quite a good job of keeping their walled garden well controlled and secure. I barely see anything of their website, but others are willing to jump through hoops just to be in there (e.g. sending in copies of passports). You know, these people will put up with quite a lot of abuse.