Almost everything found online is copyrighted, and how it may be used is up to the copyright holder (usually but not always the person who posted it) and varies from one item to the next. Best bet is to never copy and paste from anywhere without citing the source (such as linking to the original article). Even citing the source doesn’t always mean you have permission to share the information.
Having said that, most companies encourage social network sharing nowadays. If their article or the web page has social sharing buttons, the easiest way is to use available sharing buttons to post it on your social network. Those links imply that the information can be shared, and usually their own sharing buttons are preset for exactly how they want it shared. Or copy the full URL and paste that into your own status update to share the information – this allows the full article to be linked, which helps keep the original author’s copyright intact.
Some other tips that will help you: the difference between a true professional and some who is not, and why your professional needs to understand copyright.
If you ever have a question about sharing a specific article or something from a specific website, you always have to check with that company to see what they allow.
Note I am not a lawyer, and definitely not a copyright lawyer. A little common sense will go a long way – best bet is to assume everything is copyrighted, and only use it if you know you have permission to do so. When in doubt check with the person who owns the image/article you want to use … or your attorney.