Comment by brianwhitman

EN (which powers the Rdio create station feature that rolled out recently) has similarity data and popularity data for songs & artists natively.

Popularity of a single song: http://developer.echonest.com/api/v4/song/search?api_key=FIL...

List of songs by an artist ordered by popularity

http://developer.echonest.com/api/v4/song/search?api_key=FIL...

Since we have Rdio in our Rosetta ID space, you can natively use Rdio IDs and get them back in your calls:

http://developer.echonest.com/api/v4/song/search?api_key=FIL...