I guess the reasoning would be that some hackers probably have found vulnerabilities they'd rather sell on the black market for 50K than sell to Facebook for 5K.
How do you recoup 50k on FB? Not a theoretical "I'll hack Tom Cruises' pictures and blackmail him" but an actual demonstrated business model.
Also note that the majority of government entities can just legally request information.
If anything smaller governments without in-house vulnerability research would be more willing to pay large amounts.
>Also note that the majority of government entities can just legally request information.
The kind of governments that would be interested in exploiting Facebook probably aren't the kind that could legally request the information in the first place.