Privacy law is a hot topic these days. In both the public and private sectors, employees’ privacy rights are governed by whether or not the employee had a reasonable expectation of privacy. In the private sector, this is a common-law tort for the invasion of privacy. In the public sector, it becomes a constitutional question under the Fourth Amendment.
An interesting case from the District of Nevada earlier this month demonstrates the Fourth Amendment analysis in the context of social media. In Rosario v. Clark County School District, a student brought a Fourth Amendment claim after he was disciplined because of tweets (i.e., posts to Twitter), he had made that were critical of the school and its faculty. The student argued that he had a reasonable expectation of privacy in his tweets because a limited audience (his followers) viewed or read his tweets.
The court rejected this argument, explaining:
When a user with a public privacy setting tweets a message, he or she intends the message to be heard by the public at large. It just happens that typically the only people that read the tweet are the users' followers. A tweet from a user with public privacy settings is just a twenty-first century equivalent of an attempt to publish an opinion piece or commentary in the New York Times or the Las Vegas Sun. When a person with a public privacy setting tweets, he or she intends that anyone that wants to read the tweet may do so, so there can be no reasonable expectation of privacy.
This reasoning is consistent with the idea that there is no “limited-audience” expectation of privacy. In other words, once an individual publishes information to another person, absent a lawful privilege, such as the attorney-client or spousal privilege, the individual has no reasonable expectation of privacy in that information. Once it has been published to another, it is no longer protected.
The Rosario court affirmed this principle, stating:
A Twitter user with his or her privacy setting set to private has a more colorable argument about the reasonable expectation of privacy in his or her tweets than a user with a public setting. However, even with a private account, the user is still disseminating his postings and information to the public, and they are not protected by the Fourth Amendment.
The court elaborated that, when a person “tweets on Twitter to his or her friends, that person takes the risk that the friend will turn the information over to the government.
One of the cases cited in the Rosario decision similarly supports this holding. In United States v. Meregildo, the Southern District of New York held that the government does not violate the Fourth Amendment by gaining access to a Facebook page through a cooperating witness who is Facebook friends with the account holder. 
Thus, the Rosario decision is consistent with the case law. Unfortunately, many courts have not been quite this consistent when determining whether a reasonable expectation of privacy exists in social-media content in the context of discovery disputes.
 No. 2:13-CV-362 JCM (PAL), 2013 U.S. Dist. LEXIS 93963 (D. Nev. July 3, 2013).
 Id. at *15
 Id. at *16 (internal quotations omitted).
 Id. (citing United States v. Meregildo, 883 F. Supp. 2d 523, 526 (S.D.N.Y. 2012) ("When a social media user disseminates his postings and information to the public, they are not protected by the Fourth Amendment.").
 883 F. Supp. 2d at 526.