Earlier there was a lost of rumors from Facebook monitors communications of it's users, sells their personal data, etc. This april Facebook's assignee posted an official report with 225 pages, exposing which data was collected by Facebook
This list includes:
- Types and names of files on disks
- Browser extensions
- Purchases on different resources
- Mouse motion
- Pages of users which were entered from your device
- Battery level
- Your history
- Apps on your device
Besides Facebook muster such information like when and where was made photos of a user or spots where user have used Wi-Fi or mobile network which were close.
If you are visiting this network decently often, you would be better with the use of a proxy for more effective work.