First, you could just resign and choose not to participate in ongoing violations of privacy. By not giving them your help, you are already making a small difference.
Second, you could threaten to resign because of this. Better, you could get together with others of like mind and threaten to resign en-masse. If enough people do this, upper management will take notice. It's just a matter of how many Google and Facebook employees have the will to do it.
Third, when privacy issues come up, employees could try to influence their companies to err on the side of privacy. In companies this size there are always many situations which aren't guided by explicit policy or direction from on high, and where employees can use their own judgment or advocate for one course of action over the other. This is where people who care about privacy can make their voices heard.
Finally, Google and Facebook employees are smart, creative people. They are more than capable of figuring out other ways they can make a difference -- if these issues are important to them. The main question is: are they?
With all the media attention around Facebook and Google privacy violations it's becoming harder and harder for their employees to stand on the sidelines and claim ignorance or pretend there's nothing they can do. By not doing anything they are choosing to continue enabling these companies. They could act instead.