If you are asking which I'd pick, I'd pick google. However, I'd specifically want to work on/with google earth. I think google earth is awesome and would like to know more about it, the technology behind it, the geosciences that make it possible, etc...
If you are trying to figure out which offer you should take, you should ask yourself what you would rather be doing. Would you rather be working in the world of searching? Or social media? It might be worth asking both of the companies for more information on what roles and responsibilities you would be expecting, and what department they might stick you in.
I wouldn't stress it as the most important decision of your life -- after all, if you work at google and decide you don't like it, you wouldn't be the only person to apply at facebook! ;)
1) Facebook is cooler, go there now. Or:
2) If you join Google and you find it's not for you, just catch the Google-to-Facebook bus.
Personally...I'd rather work for Google.
Facebook is a good choice too but learning, there, will be different.
Could you elaborate on how working for Google and what you learn there is going to help you working on your startup ?