"Someone Like You" wasn't released as a single or through iTunes or anything like that before the album, but the album was promoted and previewed through the
21 Days of Adele, and some of the events included videos of live performances of some of the new songs (I think the Live at Largo videos were posted during this time), so not only had she debuted them live at a show, but posted videos of the performances, so people heard the songs before. And then she allowed fans to stream
21 for free on NPR before its release.