Earlier quoted context omitted.
It doesn't matter what you have in your password manager. If you're not a US citizen, they can demand your facebook access. If you don't have it, can't unencrypt it, or just feel like telling them to fuck off doesn't matter. You have no legal right to enter the country except with their permission, and they may well tell the person who didn't put their facebook passwords in the password manager to get back on a plane…
So basically, as a non-US citizen, if you don't bring a device that can log into your Facebook account, they might deny you entry? Of course rhey might deny you entry for any reason, and them caring so much about your social media accounts is probably a sign you're on a shitlist somewhere, so maybe it doesn't matter so much. I wonder, though, if you don't have FB etc. accounts, do they believe you?
A large majority of people do have FB accounts, and even care about their FB accounts as illustrated by their worries about providing access to them.
For most people they wouldn't bother to look, only a tiny fraction of travelers get their devices checked. And if you really don't have FB etc. accounts, they may believe you - I have little reason to doubt that; I feel that if that was their policy then we'd hear about many such cases.
However, if you claim that you don't have a FB account and their automated system (seeded by the information submitted by the airline before your arrival) shows a FB account with your face, then that seems a sure way to get turned back at a border.