You have the right not to post! Anything you post can and will be used against you in a court of law!
There are a lot of misconceptions out there regarding social media. Some people think the First Amendment applies to private business (it doesn't) or that simply because a service is "free of charge" that means it must come without cost (the price may be your mental wellbeing [1]). One of the most damning mistakes a person can make is assuming that because something is their "personal" or "private" social media account than law enforcement cannot use the contents thereat against you without a warrant to obtain a copy of it.
Sadly, nothing could be further from the truth
Reasonable Expectation of Privacy
To understand why your social media posts can be accessed by law enforcement and used against you in a court of law, we need to look back at a Supreme Court case from 1967; Katz v. United States.
Katz v. United States was a landmark decision of the U.S. Supreme Court in which the Court redefined what constitutes a "search" or "seizure" with regard to the protections of the Fourth Amendment to the U.S. Constitution. The decision expanded the Fourth Amendment's protections from the right of search and seizures of an individual's "persons, houses, papers, and effects", as specified in the U.S. Constitution, to include as a constitutionally protected area "what [a person] seeks to preserve as private, even in an area accessible to the public". The case introduced the Katz test to determine a reasonable expectation of privacy, defined as a two-part test written in a concurrence by Justice John Marshall Harlan II. The Katz test has been used in thousands of cases, particularly because of technological advances that create new questions about cultural privacy norms. [2]
In short, Katz found that the Fourth Amendment protection against warrantless searches does not apply to information that a person knowingly exposes to the public. However, the Fourth Amendment does apply, to information that an individual attempts to preserve as private [3].
Unfortunately for the individual who admitted to a crime on Facebook or tweeted about a possible felony, the Court would find that an individual who knowingly exposed the information to the public and, therefore, that it is admissible in court no matter how police or prosecutors came to possess it.
Individual Attempts to Preserve Privacy
Theoretically, it is possible to post content onto social media than a person could argue still is done in a way to "attempt to preserve privacy" if we use the language of Katz. For example, what if someone creates a private YouTube playlist with videos on how to pick a lock. Another example might be if an urban explorer, people explore derelict urban structures, created a private Facebook photo album of pictures taken while trespassing. Could these materials be introduced in court if a District Attorney or Police Department came into access to them? The sad truth, like many matters involving social media, the issue has not been truly addressed yet.
Entrapment
Entrapment, in its simplest form, is when someone tricks another person into committing a crime. The example we are all most familiar with is from police procedural television shows where the detectives get a little too aggressive in setting up the weeks bad guy and a Judge cries foul.
Entrapment still applies on social media, as far as anyone can tell. While investigators routinely use fake social media accounts to 'Friend' or 'Follow' possible targets, they cannot use those accounts to encourage criminal behavior.
Conclusion
I have three rules I live by; the camera is always on, the gun is always loaded, and your wife will always finds out. When posting on social media, keep these rules in mind but exchange 'wife' for 'innovative law enforcement professional'.
One should never break the law but if one does, one should definitely not post about it.