I think part of the reason for no revenue is the stumbleupon traffic.
My experience is that stumbleupon does not really create any revenue. It is great to get your name out there though.
I think you need to wait for some time until you get more targeted traffic via google, etc. Then you will start earning some money.
But it seems that your site is popular so keep up the good work.
Hope this helps.
Fabian
