How does the HTC First compare to other mid-range HTC devices?

In all the talk about the low sales of the HTC First (the so-called Facebook phone), there’s one bit of information, and one question, I’ve not seen answered How do the sales numbers of a mid-range Android device on a US network that runs the Facebook overlay compare to sales of a mid-range Android device on a US network that runs HTC’s own UI layer? Perhaps the HTC First sold in the same rough range as other mid tier Android devices targeted at regular consumers?