There's a little something called "incentive". Sure Roku doesn't write the apps, but they could incentivize facebook and even collaborate with them on it.
The other problem is Apple and Facebook for iOS. While many iOS apps have a cast button, like YouTube and Netflix for example show destinations like my xbox one and two Roku's... the Facebook IOS app does not. Facebook is just using iOS's built in cast library which only seems to work on Apple TV or an AppleTV emulation program you can install on a PC (no help to roku). Even installing the Apple TV app on Roku and ensuring its launched... it still does not show up as a destination for casting to.
So its a pretty tough problem with at least two bullheaded companies (Facebook and Apple). Roku I'm sure - at least in theory - would be more than happy to have a facebook app... but they can't write it themselves. They would probably be sued by Facebook for reverse engineering and using their intellectual property. All Roku can do is incentivize facebook. Provide them with all the top notch developer support they need. Provide them with documentation and prove that development will have reach. Maybe that means disclosing sales figures, number of live Roku accounts, demographic information... the kind of stuff Facebook drools over.