If you use Facebook, you're a willing participant.
That being said, the website tows the line in legality. "Bait and switch" might apply here.
Just like a few other companies, it tries to trap it's users into it's world, to use and manipulate their data and communications.
For your reading enjoyment:
1). FB intercepts and re-routes direct personal emails secretly through it's own servers:
http://mashable.com/2012/06/25/facebook-email-address/
2). FB uses friends to sell you stuff:
Facebook Must Allow Users To Back Out Of Personalized Ads [Class Action Settlement]
3). FB facial-recognition is spilling out on Main Street:
We're watching: The camera that can recognise you from your Facebook picture every time you walk into a shop | Mail Online
4). FB tracks (i.e. "stalks") you on the web even after logging out:
Facebook: Tracking Your Web Activity Even After You Log Out? | News & Opinion | PCMag.com
5). There's plenty more:
Facebook Is Normalizing Creepy Behavior