[gorner]

Not a lawyer, but I think you're more or less right about North American trademark law as well. Trademarking a common word is fine (so long as it's not inherently descriptive of the actual product – i.e. trademarking "Paper" with respect to reams of paper would never fly), but FiftyThree would still have to show that "Paper" is sufficiently distinctive in the software market that anyone discussing "Paper" in terms of mobile apps instantly thinks of their product, the same as "Apple" being distinctive in the computers / electronics industry. As big as their app is already, as others have noted other apps have that name too, so 53 would have difficulty claiming exclusivity on "Paper".

Just did a quick USPTO search and it looks like 53 hasn't attempted to register a trademark for "Paper" itself – but they have registered "Paper by FiftyThree", which would more easily pass the bar of distinctiveness: http://tsdr.uspto.gov/#caseNumber=85622666&caseType=SERIAL_N...

Having said that – I do wish Facebook had picked a name that wasn't already associated with such a well-known app.

Disclaimer: I have a dev friend who works at FiftyThree but their IP strategy is not something we've discussed.

edit: spelling / grammar