If 53 didn't register Paper as a trademark, they are pretty much SOL, right? If they did, it's pretty clear that Facebook's property creates confusion in the market. Trademark law seems pretty clear here, but IANAL.
IANAL either, but my understanding is that, in the US, they don't need to file a trademark as long as they can prove they were using the name in commerce first.
As far as international usage, that's a different story.
As far as international usage, that's a different story.