Is this common knowledge? Sounds like incredibly poor practice to sniff emails & address books without prior permission or at the very least, transparency.
They all ask for permission and most people just tick the box. Linkedin lets you do the same thing. People give them their passwords to their email accounts willingly. Looks to me it's a pretty common practice but I'm no expert on social websites. I wouldn't do it but that doesn't matter, there's always plenty of people who will.
its a "let uns look at your contacts so we will find other friends of yours on facebook"-feature. and a lot of people (sadly) dont care about this.