The licence is listed at the bottom of the product's description:
LICENCE -- Do whatever you'd like with these. You may use use them for both non-commercial and commercial use as long as you give credit to the original owner of the logo (i.e. to use the twitter logo, make sure you link back to twitter), make sure to not use any of these logos for anything other than to link to their rightful owner. This does not include, however, the joystick logo. You may use the joystick logo for anything you'd like. No credit back to me is required (although it is greatly appreciated).