Seems like you'd build up a great dataset for training an automated hum -> song classifier if this gains traction.
I have great melody memory, terrible title and lyrics memory, and good pitch, and it has never helped me the many times I've tried to figure out what song I got in my head.
My memory might be failing me though. There was another competitor to shazam with an orange soundcloud like logo that also has song id. It could've been that app.
Edit: It was soundhound and it supports humming https://play.google.com/store/apps/details?id=com.melodis.mi...
Hummed the riff and had it in no time.
You'll note that you need to define a sequence of similar notes as 'down' or 'up' to make it work.