The word "fortnight" became less common than the phrase "two weeks" in American English in 1906, but persisted as the more popular term in British English until 1977.
I'm not drawing any particular conclusions here. Just thought it was interesting. I wondered when the word "fortnight", so common in the writings of Jane Austen and her contemporaries, went out of fashion. Take a look at this for reference. Note the 1906 crossing of the green and orange lines. Those represent the two terms in the American English corpus. Also note the crossing of the blue and red lines in 1977. Those lines represent the words in British English publications.
I say we make an effort to revive the word in America within the fortnight.
See what I did there?