It is uncertain what kind of and to what extend Facebook stores data personal data and how this data is used. However it is known that Facebook store more data than the user enters knowingly and directly; also the movement and the actions of the users are monitored - possibly consistently.
Facebook also learns about
- the most important facts about the computer system used when logging into the social network - the IP address, type of the processor, browser brand and version including installed plug-ins
- every visit on websites that have a "Like"-button installed. The IP address is transmitted. If the user is a Facebook user and is logged into Facebook on the computer used, the user can be identified.
This enables Facebook to analyze preferences, affections and contacts of their users in great detail, even beyond the immediate use of the social network.