A "face book" is a generic term for a student directory.

Back in my day we just called it the student directory. Didn't have photos in any I'd seen as a kid/college student, those were only in yearbooks.

Not just a Harvard thing. A "facebook" or "lookbook" was a pretty common term both in and out of academia. It's still used in the fashion industry.

If facebook were that common, trademarking would have seemingly been a bit more difficult.

Having photos in student directories doesn't seem like a common thing.

Commonality isn't always a roadblock to trademarking.

Windows and Apple are two other examples. It definitely restricts the scope of the mark, but websites were new in all industries, just like software was when Microsoft and Apple got their TMs.

Depends on your criteria for common, but it was certainly not unique. We had them at the non-Harvard university I attended and IIRC, it was colloquially called the "freshman face book".