I wonder, though, if you don't have FB etc. accounts, do they believe you?
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.
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.
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.
US citizens have it better in at least that one regard. They can't keep me from entering the country. They can confiscate my laptop, but if I don't want to give them facebook access, there's not much they can do to stop me. They can of course add me to some list that makes my life miserable every time I fly, but there's no constitutional provision that allows preventing a citizen from entering the country.