That metric primarily captures efficiency and activity of the app, rather than efficiency of the DC. A hello world app would look amazingly efficient by this measure, because it doesn't do anything, so you're basically just capped by how many connections a box can support. A video processing app would look amazingly inefficient just because it's doing so much work.

An infrequently-used app also looks better by this metric than a frequently used app. The service behind the mobile weather app you use might look more efficient than Facebook, just because you use it once a day instead if a dozen.

Disclosure: Microsoft employee, not involved in our data center designs.