According to his biographer Laurence Bergreen, "Everybody's Doin' It Now" was Irving Berlin's attempt to exploit the early 20th Century dance craze in America by "writing a generic dance with a deceptively casual title".
It has also been suggested the title - and the song - is a double entendre, but Berlin himself said, "It was an idea out of the air. I wanted a dance song; everybody was doing it. I just sat down and wrote the thing as it was. It was the dance craze put to music and words."
The idea worked, and the song became "the standard tune of the dance craze".