Mark Zuckerberg says extent of opioid crisis was biggest surprise of US tour

Speaking about his 30-state tour that sparked rumors of a presidential run, the Facebook CEO added: 'We have a responsibility to remain optimistic'

The "biggest surprise by far" from Mark Zuckerberg's listening tour of America is the extent of the opioid crisis, the Facebook CEO said on Friday.

"It's really saddening to see," he said, referencing the 64,000 people who died from drug overdoses last year.