libimage-magick should be sufficient, however I don't know how well that runs on androids and iphones.
It smells like a NIH project to me, but then again there might have been some very special requirements that facebook needed that was not already available in other libraries.
At Facebook's scale, the choice of whether to resize/crop/compress images on devices or at the backend must be a complete no-brainer. Why would you stand up additional datacenters instead of just burning a little more of your user's (your "product's"?) battery?
It smells like a NIH project to me, but then again there might have been some very special requirements that facebook needed that was not already available in other libraries.