Want to know who logged into you Facebook account without your knowledge? Simply click on Security and Login in your Settings and check out where you’ve logged in.
Here you’ll see all the sessions currently using your Facebook account. Don’t worry if you see more than one. Look a little closer: you’ll probably see all your sessions from both your cellphone and your browser.
Facebook can show you what device was used and from where someone logged in for the session. If you see anything suspicious, then click on the three little dots beside that session and log out of it. If you click on „See more” below, you will be able to see a lot more sessions and you can even log out all of the sessions with a single click of a button (except your current session). On certain devices (smartphones and tablets for the most part) every single time you enter the Facebook app it’ll register as a new login and it’ll show up as a new session.
If you are logged in to Facebook, here is the link: https://www.facebook.com/settings