CBP ESTA and Visa Waiver Program VWP FAQs add Social Media https://www.cbp.gov/travel/international-visitors/visa-waiver-program/visa-waiver-program-improvement-and-terrorist-travel-prevention-act-faq
While the Social Media questions on the ESTA / Visa Waiver Program (VWP) Application are optional and are not required to be answered. U.S. Customs and Border Protection (CBP) updated their ESTA / VWP FAQs to advise that social media profiles are subject to review at admission to the United States. As it may with all persons arriving at a port-of-entry to the United States.
All persons arriving at a port-of-entry to the United States are subject to inspection by U.S. CBP officers. CBP officer's border search authority is derived from federal statutes and regulations, including 19 C.F.R. 162.6, which states that, "All persons, baggage and merchandise arriving in the Customs territory of the United States from places outside thereof are liable to inspection by a CBP officer." Unless exempt by diplomatic status, all persons entering the United States, including U.S. citizens, are subject to examination and search by CBP officers Information about CBP search authority may be found at https://www.cbp.gov/travel/cbp-search-authority.
Additional information regarding the inspection process is located in the Code of Federal Regulations, see, e.g. 8 CFR 235 Inspection of Persons Applying for Admission.