Stewart Baker writes:
The third-party doctrine of Smith v. Maryland, 442 U.S. 735 (1979), is getting a bad rap from libertarians of the left and the right. Smith holds that the police don’t need a search warrant to get information about me from a third party. If I keep a diary in my desk drawer, the police must get a search warrant based on probable cause if they want to read it. If I leave the diary with my mother for safekeeping, though, the third party doctrine says that the police only need to serve her with a subpoena to get it. The same is true if I store the diary in the cloud with Google Drive or Dropbox. If it were on my computer, the police would need a warrant to read it; in the cloud, they don’t.
Read more on WaPo The Volokh Conspiracy.