There’s a new internet retailer on the world’s smartphones, tablets, and PCs. It’s called Facebook. Last week, Mark Zuckerberg and company started testing a “Buy” button inside the News Feed posts and ads that turn up on the world’s most popular social network, letting users instantly pay for goods and services from other merchants without leaving Facebook. The button provides an added level of convenience for those who spend so much of their time on Facebook already, and it marks the beginning of a new stage in the company’s evolution into a significant money-making machine. The button should come as no surprise. Inside the company, a man named Nicolas Franchet has long worked as Facebook’s global head of retail and e-commerce.