Step 1.) Login to facebook from a computer.
Step 2.) At the top right corner of the screen will be a down arrow icon. Select that icon.
Step 3.) Click “Settings & privacy”.
Step 4.) Click the settings button.
Step 5.) Look at the far left menu column and find the option “mobile”.
Step 6.) Once clicked you will then see a link called “lost your phone”.
Step 7.) You will now see a popup message with an option to “log out on phone”. Select this option and you will be logged out of your lost or stolen phone.