Google doesn't provide feedback on why they reject you. For example, you could be the world's foremost expert on Erlang. But the department interviewing you needs a C++ programmer.
You'd never know that was the reason. You'd just be notified that you weren't called back for another interview and asked to please check back for more positions in a few months.
Actually, you can extract some info from the recruiters. Just ask them, tell them you need it to prepare better next time, tell them you know the reason but want to cross check etc. They will tell you
I was just rejected by Google. They will give feedback -- in my case, the recruiter said that they were worried I didn't seem to care whether my code would actually work as written.
