Music discovery...HELP!!

Assume I'm signed up for the premium versions of any pay service, but don't ignore free services either. Is there any product/website in existence where I can search for playlists containing specific songs? Preferably linked to Google Play Music All Access or Spotify Premium.

Example: search a song, and then filter results by "playlists containing this song"

Bonus points if you can find out how to browse user generated playlists at Google Play Music All Access. The official support explicitly and in no uncertain terms says you can make and subscribe to public playlists, but it mysteriously fails to mention how anyone can find and listen to them...