Google is not going to sell every g*damn email you ever sent for $$$, but Facebook will and does. So there was never as much profit there for Google as for Facebook because Google was run more ethically.
Never say never. Companies pretty routinely do surprisingly underhanded/unethical things once they begin to experience financial difficulties, if not on their own then under pressure from investors.
Wait, how does FB have my sent emails? I'd think sent emails were one of the few things they don't have access to
Did you miss the time that google was showing ads targeted based on the content of your emails?