Hi,
Thank you for posting your query Microsoft Community. I will be glad to assist you.
1) Are you facing the issue only with Facebook app?
2) How did you come to know that some one is using your account?
I suggest you log into the other account in different computer and check if it helps.
Also, try this method: If
you have any problems with an app:
Run the App Troubleshooter and check the troubleshooter report:
The Apps Troubleshooter is an automated tool which we have developed to address a number of known issues that can prevent the
Apps from running properly. This troubleshooter automatically fixes some issues, including suboptimal screen resolution, incorrect security or account settings etc. You can click on this link to download and run the Apps troubleshooter.
Hope this helps in resolving the issue. If issue persists, do get back to us. We will be happy to assist you.
Thank you.