The Internet is public. Remember that. So even when you think you are posting something privately, once it is out into the Internet ether, it is out of your control and potentially can be used against you in the future. This is especially important in a job search. Even though a future employer (assuming they are not the government) may not be able to access your private posts, it is common to begin including coworkers as future friends. They will have access to your stuff. Keep it clean.
With that in mind, here is a list of things you should never post on social media:
The language you use on social media is usually the same type of language you'll use when casually interacting with coworkers. A minor four-letter-word once in a while isn't a crime, but do you want that to be part of who you are, even among friends? Keep it clean. Better yet, eliminate profanity from your online and offline vocabulary.
2. Abusive Content
Social media can be a great place to share opinions, but if your feed is full of arguments and rants, you may come across as an overly negative person, not the kind of employee companies want in their office.
3. "Adult" Content
A snapshot on the beach is probably fine but it's alarming how many people think nothing of posting photos of themselves in their underwear (or less!). The Internet is a public place, even when you think you are sharing privately. So, before posting a selfie, consider if you are OK with being seen like that in the office.
4. Illegal Content
Not only could this get you disqualified for a job, it may land you on America's Stupidest Criminals. Worse yet, you could be arrested if authorities see credible evidence of certain kinds of illegal activity. Try explaining THAT to a potential employer during background checks.
5. Offensive Content
Racism, sexism, and other forms of discrimination should have no place in your life. Even if you're joking, think twice before sharing something that could be seen as bigotry. And then don't share it.
6. Negative opinions about your job/employer/boss/professor
A comment about how you you're your job could cause future employers to wonder if you really like the work you are doing or not. A comment about your "stupid boss" could cause your application to simply disappear.
7. Drug related content
Some employers may require you to pass a drug test as a hiring condition. You won't even get that far if you share this kind of post publicly.
8. Poor grammar
Forget about dangling participles and comma splices. If you don't know the difference between "your" and "you're", it's time to learn. Communication, and written communication in particular, is an important part of professional life. If you can't communicate effectively, employers won't want you to communicate for them.
9. Poor spelling
Like #8, poor spelling makes you look unintelligent. Spell check helps but how many typos do you let slip through when you post on social media?
No, it's not OK to threaten others, no matter their position. Wishing someone ill, whether it is a friend, family member, colleague, politician, celebrity, whatever. If you don't like someone else, social media is not the place to be hurling threats of any kind.
Remember that social media is public, even if you restrict it to the private settings. If you are ever in doubt of this, think back to all of the private social media posts that were made public when someone commits a crime. The government (and other agencies) may have access to this in advance of employment, so keep it clean. And, even after you start, you will likely friend your coworkers, who will have access to your past posts, including that wild party where you were tagged. Keep it social, yet keep it clean.