They can drag the investigation for as long as possible, then pay a few hundred million to kill the deal. Killing a competitor (which honestly has 100% userbase overlap with FB anyway) and cockblocking Twitter and Google at the same time.

It sounds crazy, but remember when Google bid 4 billion on wireless spectrum just to create a better environment for Android?

It was rumored that Google et al were also interested in Instagram.

So a billion dollars sounds pretty cheap to prevent a competitor getting hold of a very popular and strategically important social network.