For Facebook users, there are few places to hide on the Internet.

In order to access Facebook, users must agree to a lengthy terms of service contract that outline everything Facebook has permission to do. That includes tracking users, even when they aren't on Facebook, for advertising purposes.

Facebook tracks users by placing a cookie, or a small text file, on the computers of its users. The cookie communicates with so-called pixel code installed on many popular websites.

Pixel code is often included in Facebook services that allow companies to take advantage of Facebook in several ways. Online publishers can install "Like" and "Share" buttons on content. Those buttons increase sharing on Facebook, which increases traffic to publishers' sites. Online retailers can use pixel code to purchase Facebook ads targeted to people who were just about to buy something, then got distracted and later ended up browsing Facebook.

The pixel code also lets Facebook know what its users are looking at on the Web. That information is invaluable to Facebook, because it gives the company a better idea about the interests of users. That increases the effectiveness of Facebook's targeted advertising, which means Facebook can charge companies more for ads.