To log out of Facebook on all devices at once and ensure that you’re securely signed out of your account, you can follow these steps:

1. Open Facebook: You can do this on a computer or using the Facebook app on a mobile device.
2. Click on the Down Arrow: In the upper-right corner of the Facebook page (on a computer) or the three horizontal lines (on mobile), you’ll see a downward-facing arrow or lines. Click on it to open a menu.
3. Go to Settings & Privacy: In the menu, you’ll find “Settings & Privacy.” Click on it to expand the options.
4. Choose Settings: Under “Settings & Privacy,” you’ll see “Settings.” Click on it.
5. Security and Login: In the left-hand menu, you’ll find “Security and Login.” Click on this option.
6. Where You’re Logged In: In the “Security and Login” section, you’ll see a section labeled “Where You’re Logged In.” This section displays a list of all the devices and locations where your Facebook account is currently logged in.
7. Log Out of All Sessions: Click on the option that says “See more” or “Logged in with devices.” This will expand the list to show all the devices.
8. Log Out of All Sessions: To log out of all devices at once, click on the “Log Out of All Sessions” button. Facebook will ask you to confirm this action. Confirm it; you will be logged out of your Facebook account on all devices.