The fair use section of American copyright
code is purposely vague. At any given time, there are only nine Americans
who know what fair use is. Any guidelines
you may have heard about what constitutes fair use are merely suggestions the Supreme Court can choose to take into consideration
when coming up with their decision.
So when people assure you that their posts fall under "fair use", they are either justices of the Supreme Court or are talking out of their ass.