Facebook chief executive Mark Zuckerberg said 350 million of the company’s estimated 550 million users are on the social network’s inbox feature per month, and they send 4 billion private messages a day through its system.

He gave these statistics at an announcement today in San Francisco, where the company launched an e-mail product. The point of the numbers, of course, is that Facebook will be plugging its new version of the product into a massive pre-installed user base.

You can read our live blog of the event here.