It's the job of Marcus, a gently spoken 42-year-old French-born fintech guy, to turn a proprietary messaging app into this all-encompassing platform - essentially, an operating system on which third-party apps, and entire businesses, can be built in ways that lock them into the Facebook ecosystem. The Chinese have already shown what's possible: social media giant Tencent enables 600 million people each month to book taxis, check in for flights, play games, buy cinema tickets, manage banking, reserve doctors' appointments, donate to charity and video-conference all without leaving Weixin, the Chinese version of its WeChat app.