Facebook just sounds like a social media site, but the word facebook originates from something. I looked online and found nothing relevant to the origin of the word. Would you please shed a light?
Note: The question is about the noun facebook and it is not directly asking the origin of the proper noun Facebook (the company name).
Although, as a side note, it is mentioned that the trademark of the company is in lowercase but it starts with upper case most of the time:
The official trademarked name of the social-networking service and website is spelled “facebook,” all lowercase letters. Formal writing style—as exemplified by most news and book publishers—is to treat such names as regular proper nouns, in this case “Facebook,” using an initial capital letter. dictionary.reference.com
The noun facebook is not easily searchable in Google and Google Books (because of the vast amount of "Facebook" results). Additionally, Google doesn't offer case sensitive search.
It is not in OED also.