Having an applicant's social media handles would supposedly allow the State Department to more strenuously vet whomever it suspects of terrorist ties or national security-related danger. It's not the first time intelligence agencies have proposed requiring incoming foreign nationals to fork over their internet aliases, either: Homeland Security asked to add it last June, reflecting the increased scrutiny the DHS wanted to pay to visa applicants after the 2015 San Bernardino shooting. The department got its wish back in December, if only for those applying to the visa waiver program.
The proposal, found here, would ramp up the vetting process, just as Trump promised during his Presidential run. In addition to social media handles (but not passwords), it would require applicants to list names and dates of birth for siblings as well as up to 15 years of biographical details like home addresses and travel histories. But it hasn't been adopted just yet: The proposed changes must endure a public comment period before the Office of Management and Budget approves or denies them by May 18th.