Set the transparent bsdf to one color (solid white: 1, 1, 1...alpha is disregarded). For this shader to be truly "transparent," the color must be so.
Transparency color levels from left to right: 0, 0.25, 0.5, 0.75, and 1 (yes, there is an invisible monkey on the end).
Here is the demo blend:
http://blend-exchange.giantcowfilms.com/embedImage.png?bid=1951
And here is a link to the transparent shader in the manual:
https://www.blender.org/manual//render/cycles/nodes/types/shaders/transparent.html