This should not shock you, but just because it came up in your news feed, that doesn’t make it acceptable for school or your professor. Facebook tends to reflect you and your Facebook friends, not your academic life, and most of the articles that float around Facebook don’t have references. With that in mind, be careful.
If you find an article you’d like to use, be sure to evaluate the article and it’s source on their own merit. Some question you should ask yourself: Did this originally come from a newspaper? A journal? A magazine? If it’s from a web page, who owns or operates it? If it’s an opinion piece, who wrote it? Is the person who wrote it a recognized expert? Have they written other things? Is there corporate information? Contact information? How easy would be to write a Works Cited or Reference page for this?
If you have trouble figuring out who wrote what you’re looking at or who owns it, you probably shouldn’t use it. Like Wikipedia, you shouldn’t reference that you got this on Facebook. Cite the source it originally came from.