It just seems a bit melodramatic. Should Google Mail have been named Google Post, just because Yahoo Mail was first? They aren't operating in the same space; furthermore, Facebook didn't name it just "Paper" in the App Store, they've named it "Paper - stories from Facebook".
I do not know - there are 240 apps in Google Play store that have "paper" in name. This includes "Draw Paper", "Paper Draw ", "White Paper", "Infinite Paper" and countless others. Also at least two apps are just called "Paper" - not sure how anyone can distinguish them other than looking at company name.
I seriously doubt that 53 was the first company that used Paper in the name.