Hi there! I understand that when you log in to Facebook on your Mac running OS X 10.15.6, the app crashes. I’d be happy to help get this sorted out with you.

Just to clarify a couple of things before we dive in:
- Are you using the Facebook app downloaded from the Mac App Store, or are you accessing Facebook through a browser like Safari or Chrome?
- If it’s a browser, which browser and version do you use?
- Does the crash happen immediately after logging in, or after some time using Facebook?
- Have you tried any steps to fix this already?

Once I have that info, I can guide you through some tailored troubleshooting steps. Generally on a Mac, crashes can be resolved by some simple steps like restarting the device, updating software, clearing cache, or reinstalling apps depending on the context.

Looking forward to hearing from you!