4 February 2015
The change enables it to gather data from activity across the internet, as well as the normal data it gathers on information you and your friends have added to the site. It also allows the site to pass on that information with its other branches, including Instagram.
Facebook does not share data with WhatsApp or vice versa, the company said.
The company said in November that the change would come into effect, when it mentioned that it would be implemented on January 30. Facebook says that it showed notifications to users, and sent them emails, informing them that the change was coming into effect.
It also ran a seven day comment period, allowing users to discuss the change.
Facebook says that the data used will help personalise advertising and stop users being shown information that they don't want.
Advertising is key to Facebook's revenues, and drove the surging profits that the company reported last week.
Users can opt out of some parts of the new policy by customising privacy settings. The site has a special settings page for ads,  where users can opt out of advertising that is shown on third party sites or is selected by comparing you with your friends. Adjustments can also be made on mobile devices.
To opt out of the site tracking your use across the internet, Facebook asks users to opt out through the European Digital Advertising Alliance, in the UK and Europe, or its equivalent in the US. Going through that service will allow users to opt out of the tool entirely, and once they have done so the preference will stay everywhere that Facebook is used.