Today Facebook announced that they have discovered hackers have stolen 50 million access tokens. These tokens allow them to take over an account without having to login with a password. They did it by taking advantage of a vulnerability in the View As feature that allows users to see what their account looks like when viewed by others.
To solve the problem, they have logged out all the users who they believe were affected and disabled the View As feature. As often happens in these types of breaches, there is a possibility that at a later date they may find there are more people affected than originally thought.
To be on the safe side I suggest that you logout of Facebook by going to Settings and selecting Security and Login. There you can logout of all your devices at once with a single click. Alternatively, this might be a good time to get rid of Facebook all together.