Katz v. United States held the opposite: “The Government’s activities in electronically listening to and recording the petitioner’s words violated the privacy upon which he justifiably relied while using the telephone booth and thus constituted a ‘search and seizure’ within the meaning of the Fourth Amendment.”
More recent decisions suggest that the third party doctrine, according to which there is no legitimate expectation of privacy in information voluntarily turned over to third parties, is unlikely to apply to private messages on Facebook: