At Facebook, we are using a similar technique to crop images. The part that is different is that instead of using the center of all the faces, we find the viewport that maximizes the number of faces to be displayed. http://blog.vjeux.com/2012/image/best-cropping-position.html
That's going to be the next step for these as requests come in for images of low aspect ratio which generally just become resized smaller instead of cropped around a face.