The appearance that there is more "human facial variability" than there is to some other species is possibly an artifact of human perception. Something like or perhaps just the same natural biases that produce the "same race effect" (having more facial variability) or "other race effect" (looking all the same, at least to someone who grows up in an environment without many people of such races interacting at an individual level), only with species.
As a half-ass evidence, sheep seem to be able to recognize their facial individuality (and recall faces for two years!), something I think most persons wouldn't be able to do without strainig specifically for it for a good while:
http://abcnews.go.com/Technology/story?id=98209&page=1
And I believe there's no way a human would recall an individual sheep face for two years, unless perhaps someone with those freaky perfect memories that seem to never forget anything.
I don't mean that the facial variability would be the same for any species or that other species necessarily have more facial variability, only that we are not naturally equiped to evaluate that there's a significant difference just by casual observation.
And as a sort of addendum to the study linked by Saturnine Pariah, the faces on the human lineage have become more "boring" actually. Here's a good summary of the same study:
http://www.sciencedaily.com/releases/2012/01/120111223744.htm
I haven't read the actual study, but I think it may not address so much the main question of this post, as perhaps, despite of less facial adornments, we could indeed have more "boring" variation at an individual level. The impression I have from what I've read is that the "facial complexity" that study deals with refers more to, or exclusively to facial features evolved for species recognition/isolation rather than intra-specific individual recognition. Perhaps such facial adornments even make it a bit harder to distinguish individual variation, so it could be an additional selective pressure on more social species, besides facillitating the use of facial expressions.
