What if a different robots.txt is being served up for the real Googlebots?

EDIT:

Based on the comments in their robots.txt it appears that they are whitelisting certain robots. You would have to apply for your robot to crawl their site at https://www.facebook.com/apps/site_scraping_tos.php

They probably serve up unique generated robots.txt based on whitelisted robots. You'd never know what rules it contains unless you are a whitelisted robot.