Ah, but the seeds were there. He already liked the idea of using bulk user data for analysis purposes
I read the "vow" as a promise to collect massive amounts of data.
As for the project - "Zuckerberg and friend Adam D’Angelo created an MP3 player that would keep track of every song the user played on a computer and, as it learned what the user liked, would begin to make playlists." https://www.thecrimson.com/article/2003/10/23/not-so-artific...