"the US government has proposed to ask all visitors to the US for their "social media identifiers" to use in "vetting" travelers. The question would be on the online ESTA application (for citizens of countries
“the US government has proposed to ask all visitors to the US for their “social media identifiers” to use in “vetting” travelers. The question would be on the online ESTA application (for citizens of countries in the US Visa Waiver Program) and on the I-94W arrival form for visitors from other countries.”
See: Wanna be Facebook friends with U.S. Customs & Border Protection? (Papers, Please! link)