Copyrights can expire after a long time, putting something into the public domain, and there are some fine points on this issue regarding older copyright law versions. Note that granting something to the public domain is a complete abandonment of all rights. If your work is PD, other people can even modify one byte and put their name on it. Intent, and damage to the commercial value of the work are important considerations. The first is probably fair use, the others probably aren't.
Fair use is usually a short excerpt and almost always attributed.
Fair use isn't an exact doctrine, either. The court decides if the right to comment overrides the copyright on an individual basis in each case. There have been cases that go beyond the bounds of what I say above, but in general they don't apply to the typical net misclaim of fair use. Facts and ideas can't be copyrighted, but their expression and structure can. You can always write the facts in your own words.