I want to continue discussion started in this question "Username and/or Password Invalid" - Why do websites show this kind of message instead of informing the user which one was wrong? but due to my reputation I need to create a new question.
So I wonder why Google and Facebook definitely say what was wrong: email or password while some people think this can be security issue?
Some thoughts about "this is dangerous because hacker will now that the email is valid":
I think almost every site have password recovery form with only email field in it so it is pretty easy for hacker to know whether some email exists or not. He will just enter the email in recovery form and wait for answer "Recovery email was sent" or "This email doesn't exists".
Yes, the genuine user will know about this non genuine recovery attempt and will probably protect his account rapidly. But in case of password brute forcing we also can notify a user about suspicious login attempts (Facebook used to send such emails after some number of unsuccessful login attempts).