It my happened to forget your facebook account open on your friend PC, There is an easy way to find out if your account is open on another device and if so close it from your PC. Just follow these steps

1. Log in to your account and go to account –> account setting –> security.

2. In the bootom you should see "active sessions " click on it 


3. You should see a list of all computers that have logged into your Facebook account, their location, and when this happened.

4. If you see an unknown  device in your list, just click "end activity" on the device and you will automatically be logged out of that device.