If you are smart, you use Gmail above all other free email services because it works the best. You probably use a bunch of other Google services as well. However, Google is a successful corporation that does not place your interests above their company interests. It is entirely conceivable that they may screw you some time, in some way. Until it happens, just enjoy the amazing shit they provide to you at no charge.
Besides, Facebook is already screwing you much harder than Google could even dream of.