To mark yourself safe on Facebook during a crisis or disaster:

1. When Facebook detects a crisis in your area, you may receive a notification asking if you are safe. Simply click "I'm Safe" on the notification to mark yourself safe.
2. If you do not receive a notification, you can manually mark yourself safe by going to the "Crisis Response" section on Facebook. Search for the current crisis and click "I'm Safe" under that crisis page.
3. Facebook will then notify your friends and share a post on your timeline that you are safe.
4. To check if friends are safe, you can go to the Crisis Response page, select the crisis, and under "Safety Check," see which friends are marked safe or not. You can also ask friends if they are safe by clicking "Ask If Safe" next to their name.

This feature is used during natural disasters, mass incidents, or emergencies to quickly communicate safety to friends and family.