I'm curious as to whether anyone has some more specifics on what FB's issues were with html5, enough that it gets the "biggest mistake" award here. Any ideas?

Facebook Taking Steps to Address Mistakes in Mobile Products

Zuckerberg, who appeared at ease while trading laughs with his interviewer, for the first time elaborated on technical struggles that have impeded Facebook from creating a user- and advertiser-friendly mobile application. The company spent too long trying to build mobile products using a programming language known as HTML5, Zuckerberg said.

‘Biggest Mistake’

“The biggest mistake we’ve made as a company is betting too much on HTML5,” he said.