Answer: Police are increasingly using social media sites such as Facebook and Twitter to investigate crimes, particularly in the area of gang activities and crimes against children (pornography). Privacy and free speech advocates have raised concerns that monitoring and investigation techniques could violate peoples' privacy rights, particularly if undertaken to monitor large segments of the population.
To be on the safe side, some police departments make it clear that they are following only those sites that are public (unrestricted), and are looking only at topics that appear to be popular and of growing importance. The theory is that videos, photographs, and other information available on Facebook and YouTube to anyone with a browser are essentially "in plain view." Under that doctrine, there is no search, which means there's no need for a warrant.
By contrast, a policeman's attempt to get access to a private Facebook page, or to block access, would probably constitute a search, taking the action into the realm of the Fourth Amendment. Officers would normally have to obtain a search warrant before undertaking such actions.
It's less clear where the legal lines fall when it comes to using another person's identity to gain access to a suspect's private profile. For example, can police use an informant's profile, which the suspect trusts, to view the suspect's otherwise private and restricted web page or account? Most people, if asked whether this type of police activity should be subject to the warrant requirements of the Fourth Amendment, would probably say yes, but courts have offered very little guidance.