But why should we believe them?

http://www.zdnet.com/blog/facebook/f...legations/4044

Quote Originally Posted by facebook;
“Facebook does not track users across the web,” a Facebook spokesperson said in a statement. “Instead, we use cookies on social plugins to personalize content (e.g. Show you what your friends liked), to help maintain and improve what we do (e.g. Measure click-through rate), or for safety and security (e.g. Keeping underage kids from trying to signup with a different age). No information we receive when you see a social plugins is used to target ads, we delete or anonymize this information within 90 days, and we never sell your information.

Specific to logged out cookies, they are used for safety and protection, including identifying spammers and phishers, detecting when somebody unauthorized is trying to access your account, helping you get back into your account if you get hacked, disabling registration for a under-age users who try to re-register with a different birthdate, powering account security features such as 2nd factor login approvals and notification, and identifying shared computers to discourage the use of ‘keep me logged in’.”
We're willfully taking part in a mechanism that allows our every move to be monitored in the name of stimulating our dopamine receptors and we implicitly trust this wont be abused in the face of obvious rewards for abusing this trust?