The same probably goes for anyone here who has made significant progress -- Google would rather buy a startup than try to compete with them.
You probably have to worry more about them producing a technology that obsoletes your app...
We know they assembled their own hardware from day 1. We know they manage their own backend and platform. They have their own version control. They designed their own buildings at various places. They are even planning to spend 2 years building their own HR tool (but you didn't hear it from me).
Given that they have to compete in a marketplace, they can't be overly irrational. But you can bet they will try to duplicate something they care about before they consider acquiring a startup. More so than Yahoo or Amazon or even Microsoft.