I would assume Facebook having one successful dev advocate has many benefits:
-if FB is going to build products with React, it is in FB's best interest that there is a healthy React community constantly building new things FO FREE, so now Facebook can easily integrate good ideas/tools built for React into internal systems. I would assume this is huge.
-basically recruit smart/productive devs to FB by interacting with React community on a regular basis
-buys goodwill with dev community
-An additional line of communication into potential customers, when you have a potential sale going on with a customer, if your dev advocate knows important technical leadership people at customer that can help
But really i have no idea :)