[ynniv]

Facebook was completely generic.

Yes, Facebook WAS generic. They would have had a difficult time enforcing their brand from the start. Once a brand becomes a household name (which is admittedly a vague metric), the rules are different. See Apple, Windows, Sun, Sharp, Digital.

[ig1]

The standard isn't "household name", but rather "acquired distinctiveness" which in the US is defined under the terms of Section 2(f) of the Trademark Act