Like most of my friends, I use Facebook on a daily level. I use the website, the Facebook app, and the Messenger app. It should come as no surprise, that Facebook keeps track of every time you visit them through any of those means. The creepy thing is, that your friends can keep track of this too.
In the web-based Messenger, it is possible to see when a user was last active.
By creating a simple service that checks Facebook every 10 minutes, I’m able to get an accurate picture of my friends’ Facebook usage. Many people visit Facebook as the first thing in the morning, and the last thing before going to bed. It is therefore possible to get a good impression of their sleeping habits (or lack thereof).