Could "Mid-level developer" might mean a super developer who's considered at the level of a mid-level manager? IE, mid-level != mid-range?
Google is a lot bigger than Facebook currently so I'd assume Facebook would be aiming to pick what they consider the "cream". The article mentions 118. Google has ~20K employees and a good portion of them would engineers.
In any corporation of that size, there are some "mid-level developer"s who are far better at what they do than their title suggests. For whatever reason, they just haven't moved up in title as fast as they've moved up in skill or productivity.
Google is a lot bigger than Facebook currently so I'd assume Facebook would be aiming to pick what they consider the "cream". The article mentions 118. Google has ~20K employees and a good portion of them would engineers.