Certainly, I'm not a domain attorney. However, as someone who runs a domain name protection company, my advice is to one of two things:

- Log into your domain registrar and delete the domain in your account. That will make it available for registration again, giving FB a chance to register it.

- Respond to them and work with them to transfer the domain to them.

This is clearly, undoubtedly a trademark domain. Any UDRP or lawsuit that is filed would most likely not result in your favor. It's not worth your time, hassle, and money to continue to own that particular domain. I'd put your money into a good generic keyword rich domain name instead. That's my personal opinion.

But if you DO decide to seek the advice of counsel, whatever you do ONLY deal with a qualified domain name attorney. Domain name legal issues require specialized knowledge. Unfortunately there are IP lawyers and others that will give you advice but don't necessarily have direct experience dealing with domain names.