Updated: US Customs Notice of Inspection of Electronic Devices Policy
Comments from an attorney who watches these issues: https://blogmsk.com/2017/10/03/tips-for-traveling-with-electronic-devices/
Excerpt:
On the September 18, 2017 Federal Register notice (see 82 FR 43556), U.S. Citizenship and Immigration made clear it will now routinely require those applying to enter the U.S. to provide social media handles. As such, the obvious starting point for these tips must be a reminder that Customs and Border Protection (“CBP”) officers may require arriving travelers to provide the unlock code to their electronic devices and user names/passwords to gain access to programs, including social media accounts, so make sure all your programs are closed when you cross the border! The contents on your devices can be examined, and that is true whether or not you are a U.S. citizen, and regardless of your profession. If you are selected for such an inspection, you can expect this two page summary may be handed to you.