Because it offers a far superior user experience...for the times I find myself in self-loathing after checking Facebook for the nth time, at least I want to not be struggling with the interface.

The thing with Facebook is that I use it when I know I want to give away information. For example, when I'm at an airport about to takeoff to an unfamiliar city but I'm not sure which of my casual friends (too casual to send out an email blast) are at the destination, I appreciate the location-based feature of Facebook that detects that you're at an airport and makes it easy to autofill origin and destination. I want my location to be known and so, I might as well use the native app.