Well, there are two assumptions that we can make here:
1. With absolute certainty, we can say that the stimulus directly created a ton of construction jobs. This is obvious; barring nothing else, it put idle hands back to work building things that will be useful for future generations.
2. With less certainty, Keynesianism would predict that raising the effective demand in this way and putting demand on lots of products using the stimulus will prevent a much more catastrophic impact on the economy at large, as demand starts getting back into line.
The trend line was pretty obvious before the stimulus was passed, and the stimulus pretty obviously interrupted this trend line.