Those travelers would be violating their FaceBook ToS:
"You will not share your password (or in the case of developers, your secret key), let anyone else access your account, or do anything else that might jeopardize the security of your account." [1]
Perhaps this would give FaceBook the power to intervene and, well block their account or something.
IANAL, but even though this is true, Facebook could still ban accounts for complying.
They are a US company, and their TOS says they can ban an account for any reason or no reason at all. If they chose to ban anyone who shares passwords, even with the US government, even if they are Americans or not, they are within their rights as a US company to do so.
It's quite likely that any law promulgated to compel the production of passwords or any other authentication information would include language prohibiting a provider from terminating an account for complying with the law.
"You will not share your password (or in the case of developers, your secret key), let anyone else access your account, or do anything else that might jeopardize the security of your account." [1]
Perhaps this would give FaceBook the power to intervene and, well block their account or something.
[1] https://www.facebook.com/terms