If you've published a journal article, you want as many people as possible to see it, read it, and (hopefully) cite it in their own work.
Now websites like ResearchGate and Academia.edu urge you to share the full text of your published articles on their platforms. There are sites like arXiv and PubMed Central that will accept different versions of your article. Many researchers (especially young researchers on the tenure track) use these sites because they want to promote their articles widely.
This kind of electronic sharing is so very easy! Sending out that final PDF from the journal is incredibly simple. But should you post that final version of the article everywhere?
As an author, you signed a copyright transfer agreement provided by the publisher. That document will tell you if you can post the pre-print, post-print, or publisher's version of the article on these sites. If you can't find that document, or the legalese is hard to decipher, you can use SHERPA/RoMEO to see a summary of the journal's current agreements. More author tools, related to copyright, are available on the Author Rights page of the Scholarly Communications guide.
Your librarian can also help with these kinds of questions!