From TFA: 'Many articles characterized the change as a ham-fisted way for Facebook to push its email system, which it first announced in in late 2010. At that time the company said its goal was to integrate conversations across multiple channels of communication — text messages, Facebook chat, email, etc.
Facebook seemed surprised by the reaction. "We basically defaulted to show your Facebook address as we rolled this out, just to keep it consistent for everyone," said Meredith Chin, Facebook's manager of product communications.
"I'm seeing this whole meme around the idea that it's us pushing for people only to use facebook.com addresses," Chin said. "That was not our intention. We want people to use whatever's easier for them."'