Someday, departure and arrival forms in the US might have a new section asking foreign nationals for their Facebook and Twitter accounts. The Department of Homeland Security has submitted a proposal to add a section in those forms that reads: "Please enter information associated with your online presence--Provider/Platform--Social media identifier." According to the info published by the Federal Register, it would be an optional field and would only ask for people's usernames on social media platforms, not their passwords.
Back in 2015, DHS reportedly began working on a plan to make social media check a part of the US visa application. This proposal, however, would cover all non-citizens entering and leaving the US, even those traveling through the visa waiver program. If you'll recall, the government faced criticisms after reports came out that one of the San Bernardino shooters posted jihadist messages on social media. At the same time, terrorist organizations have been using websites like Twitter and Facebook to promote terrorist acts.
Homeland Security plans to use the social media info they gather to vet visitors. As the agency wrote in its proposal: "Collecting social media data will enhance the existing investigative process and provide DHS greater clarity and visibility to possible nefarious activity and connections by providing an additional tool set which analysts and investigators may use to better analyze and investigate the case."