A person can revoke Facebook's and Google's access. A person at least gets to read the TOS of fb and Google, we both know that rarely happens, but the possibility exists where it is completely absent in the case of the gov't.

>The 4th Amendment covers the relationship between you and the government.

Yes, of course it does. And it's because the gov't cannot scan and fish facebook for crimes to solve that people feel comfortable enough to share such information.