I don't think Facebook is likely to release a Linux based app. If they did it would likely be electron style. There also lots of Facebook apps that wrap the mobile website inside of a stand-alone "app" available on F-Droid. I also wonder what type of permissions API even exist that would allow you to view contacts as an app inside of Purism. Maybe Gnome has something kind of API already for apps to access built-in contents but this far there hasnt been a lot of proprietary software released for Linux that embeds spyware because of the low # of users and increased difficulty and general lack of distribution platform. But Purism is also really far away from being a viable platform for non-techies at this point.