U.S. trademark law uses what are called the Abercrombie classes of whether a purported brand name is 1. generic, 2. descriptive, 3. suggestive, or 4. arbitrary or fanciful, as applied in the context of the product or service for which it's being used as a brand ([1] and see e.g. [2], p. 7). The word "Apple" as a brand for computers is famously an example of an arbitrary or fanciful word in its product context. The same might not be said for the word "paper" for a medium for writing and drawing.

1. Abercrombie & Fitch v. Hunting World, 537 F.2d 4 (2d. Cir. 1976)

2. http://ipmall.info/hosted_resources/Germes_Newsletter/germes...

Are you calling "Facebook" a common English word? Because it's not.

It is if you go to a college with a facebook style directory.

http://en.wikipedia.org/wiki/Facebook_(directory)