There is no dedicated app for Facebook on Roku. Follow this guide to know how to use the Facebook app for Roku and access the Facebook Watch.
Head to the Settings on your Roku.
Go to the Screen Mirroring option on the Roku and select Enable.
On your mobile, turn on the cast option.
Make sure to connect your Roku and the mobile to the same network.
Select your Roku device on mobile and start streaming the Facebook app for Roku.
Click on the menu bar on the Facebook app and select the Watch option to access the Facebook Watch.
Contact us if you face issues with the Facebook app for Roku.