Some "Fair Use" Examples

You get a personal message and you pass it along to one other person.
PROBABLY UNFAIR, though you might still have an implied license. (For instance, if the message asks for help on a nonprivate matter, there might be an implied license to pass it along to others who might be able to help.)
You download an article from a newspaper's Web site and post it to a news group. The site carries advertising, and says "Do not send any copies of these articles to other people" (thus negating any implied license).
PROBABLY UNFAIR, since if this becomes commonplace, fewer people would access the Web site and see the advertising.
You key in an article from a paper magazine that doesn't have a Web site and post it to a news group.
PROBABLY UNFAIR, if the magazine is available on some online service (such as DIALOG or NEXIS), or if the magazine is still on the newsstands for people to buy.
You forward someone's message from one publicly accessible news group to another news group.
PROBABLY FAIR, because the message was published, and because the person posting it has no commercial interest in selling the message.
You quote a few sentences from a news article that you downloaded.
PROBABLY FAIR, because it's only a few sentences.

authors:

Larry Lessig David Post Eugene Volokh