Or, worse, they let you in, someone does a random check against NSA or whatever databases, finds out you lied and now you get deported and a permanent ban on reentry.
If I want to tell Uncle Sam or a prospective employer I have no social media then that’s the whole point. :/
Your example is what will surely happen though at the most “opportune” time, too.
Not in a day where everyone's data is being sold around for pennies [1] for the private sector, and the NSA is monitoring the entire Internet anyway and I'd wager a significant amount of money that they have direct feeds to major ISPs allowing them to cross-reference IP addresses and other forms of identifiers to real persons.
[1] https://www.washington.edu/news/2017/10/18/for-1000-anyone-c...