If you have AT&T’s gigabit Internet service and wonder why it seems so affordable, here’s the reason—AT&T is boosting profits by rerouting all your Web browsing to an in-house traffic scanning platform, analyzing your Internet habits, then using the results to deliver personalized ads to the websites you visit, e-mail to your inbox, and junk mail to your front door.

via AT&T’s plan to watch your Web browsing—and what you can do about it | Ars Technica.

Use https.  They may know which sites you visit but they won’t know any of the http fields because that is all encrypted.  Most big sites like Google and Facebook use https by default nowadays.