For banks, what's been concluded is the equivalent of a climate-change deal: an attempt to reverse potentially lethal global warming in the financial sector.

BBC: Why have we left bank reform to technocrats?

"Tesco's exit from the US is a reminder for managers of the dangers of going blindly for scale and cost leaders, the wheels of which are difficult to reverse if you need to change course to becoming a retailer known for first-class customer experience, " Prof Bhalla said.

BBC: Tesco profits fall as supermarket pulls out of US

He said Facebook is likely banking on the notion that many users won't bother to reverse the change to their displayed email, thereby leaving a Facebook address in place.

WSJ: Facebook's Email Change Riles Users