What possible interest would the government have in such a rule, and how could government agents possibly be able to know what contracts you've signed? They haven't read the Facebook ToS any more than you or I have.
There was a case recently of a NASA scientist being hassled at the border for his phone. He initially refused on the basis that handing it over would compromise his obligations to his employer. It did not work.
https://www.theverge.com/2017/2/12/14583124/nasa-sidd-bikkan...
