Why would anyone just give a site their password to their email account? And to Facebook on top of that?
Convenience. That is, Facebook - and others, like Skype - tells new users that the easiest and quickest way to find your friends is to send them your contacts so they can cross-reference the users. And that, including me not paying attention, is how all my e-mail contacts got an email from facebook where I invited them to FB. That wasn't the intent!
Also sad is the fact that BlackBerry already had a fine-grained permissions systems pre-iPhone days, but it took iPhone and Android many many versions and years before they built such privacy controls (but yeah "We care about our costumer's privacy" - Apple). And Google didn't even care about privacy back then I remember the Google Maps app for BlackBerry just prompts you "Please give us all the permissions we want or this app will just exit now." on startup, when you've denied it a permission or two.