Gaining access via fake identity to an individual's Facebook page and chats is a "search" requiring a warrant
The apparently increasing use of fictitious, sham Facebook accounts by law enforcement officers involved in sexual predator "sting" operations has been the subject of considerable criticism of late (see e.g. here, here, and here). The pattern seems to be that police officers set up fake FB accounts, posing as underage women, and then "friend" various persons whom they believe, for one reason or another, might be engaged in unlawful sexual conduct with minors.
In a recent case in Bozeman, Montana, the State's law enforcement agent posed as a 16-year old girl, arranged to become friends with the defendant, and then exchanged sexually explicit pictures with the defendant and arranged for a meeting (at which point the defendant was arrested for attempted sexual conduct with a minor). Defendant moved to suppress all of the evidence obtained through the FB impersonation, and in its recent decision, the district court in Gallatin County, Montana, agreed, holding that the defendant had a subjectively and objectively reasonable expectation of privacy in the contents of his Facebook page (given that he had chosen to use the highest available privacy settings for the page), and in the "chat" conversations that he had with other FB friends online, and that the State's use of evidence it obtained from his page and from those chats was a "search" requiring the government to obtain a judicial warrant before collecting the evidence. [Full disclosure: I appeared as an expert witness on defendant's behalf in the case].
"By its position in this case, the State is informing all Montanans that they do not have any privacy interest in their text messages, emails, or Facebook accounts. According to the State, if A Montanan wants to have any privacy interest in hs or her communication, it must be made in person or over the telephone. . . . This court concludes that i) the State's intrusion onto Windham's Facebook page and subsequent monitored conversation via Facebook chat is a search because Windham had a subjective expectation of privacy in these particular online communications; ii) this is a expectation of privacy that society deems as reasonable; and iii) the State has not justified any reason, let alone a compelling one, that prevented it from obtaining a search warrant to access Windham's Facebook page or private communications."
It's only a state district court, and some of the reasoning may be rather specific to Montana (because it provides somewhat broader protections against searches and seizures in its Constitution than the federal government provides in its) - but still, it's an important small step forward in the ongoing privacy wars.
[Views expressed here are my own - please don't mistakenly attribute them to the institutions with which I'm affiliated]