I have tested this and noticed this error when I tried to login via FB on your www site url(http://take.ms/nPyJe).
FB login on the non-www version of the site should work fine(http://take.ms/OOEJK).
In order for the www version to work, may I suggest adding the www version of the valid oauth redirect URIs in your Facebook app too(http://take.ms/dxFWv).