David Marcus, who runs the PayPal online payments service for eBay, is moving to Facebook to become head of its messaging products. This is a very big Silicon Valley poach by Facebook, which came after several discussions between Marcus and CEO and co-founder Mark Zuckerberg. It’s also a clear indication — given Marcus’ payment background — that the company intends to begin to monetize its messaging products. There has been speculation, for example, that users would eventually be able to attach digital cash to Facebook messages. But the company could use its popular messaging product in a number of different ways. The social network said it is now processes 12 billion messages on its Facebook Messenger daily. It has 200 million months active users of its app, which can be used separately from the network, but is also deeply integrated within it.