Since you're already using the last.fm api, might I suggest you add a simple scrobbling feature. If I were to use Tubalr often I would really want to be able to scrobble all the songs I've listened to through it.
I'm thinking about adding a set of features that allow you to register, login, and favorite songs. Maybe when you favorite songs they can also be "scrobbled" if your account is connected with last.fm.