Not sure, but the following suggestions came from a forum:

somewhere this answer appeared ?? Don't know who, was not on this answer spot, though, said click on F11 and it worked, the top Facebook and under that the Yahoo stream appeared and the house, the star and the gear for F/B

Swipe in from the right edge of the screen
Tap Settings
Tap Settings > Log Out
You may be asked to confirm before you're logged out.

another thought was .... try doing it from a mobile phone using the mobile facebook app instead of computer or laptop ...

That's all I found for now....trying.....

Check your FB setting, perhaps you have something set about always staying logged in. When all fails shut down completely. Clear out cookies and tracking/history, make sure something isn't open in background. hm never run into that before. Good luck

I had problems closing out my husband's fb account when he no longer wanted to do it. Took a couple days, even after searching the internet for info on closing fb. Can be a problem for sure.

Clear everything from your browser; cache, history etc..