I think the only way to interpret this that's even remotely practical to implement is that while they have the right to ask anything of anyone, in practice, it will likely be targeted. If they're asking, they probably already know quite a bit about you, including whether they think you match some social media profile they've red-flagged.

I'm certainly not suggesting that Trump isn't stupid enough to think having CBP attempting to guess whether 800,000 grandmothers every hour are lying about not knowing what Facebook is would be a plan he could seamlessly roll out, but I think any such plan would crater pretty spectacularly.

There's a big factor of subjective evaluation, but it can be quite effective.

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.

So, basically there probably going to be honest people where they would tell the truth, that they don't use thefcnkfacebook but because some scammer has created a fake profile of them would be turned back at the border. Amazing brave new world we live in...

> shows a FB account with your face

Which, amusingly, is a Facebook spammer strategy — clone someone's account, including profile picture. So even if you delete your account, there may be a spammer clone that shows up in CBP's database.