* Facebook has recently hired a number of expert language theorists and practitioners. Doing it in-house
1) Gives them something to do
2) Serves to cement Facebook's language-expertise-brand recognition and dominance.
The very fact that hiring is focusing on this group signals to me that this is an area which Facebook takes seriously and wants to be taken seriously in.
But some reasons not to use Coverity then:
* Doing it in-house gives Facebook near total control over what the system is going to focus on; they can tailor it exactly to their problem set.
* It's a worthwhile open source project, since most values of "expensive" mean "other projects won't ever use it".
* If it gets any traction as an open source project, they can draft off the work other people will put into it.