This must just be security theatre.
The problems, as I see them are that any reasonably paranoid person (criminal or not) would:
1) Not use his/her social media for anything that would arouse suspicion.
2) Alternatively, have multiple accounts, some of with which he/she could hand over to placate the authorities.
3) Not bring cell phones/laptops into the country that weren't sterilised, as it were.
I mean it's basic field craft and this has been pointed out numerous times to the authorities, which leads us to the conclusion that this isn't about finding criminals at all, but seeing to be doing "something" no matter how ineffective.
As an aside, I listened to a report on Radio 4 last week, which asserted that starting in the weeks the "Muslim ban" has been in place, the number of people of all ethnicities that have flown into the US has declined by 8%. (Compared to the year before and factoring in economic variables). The tourist industry must be loving Trump.