According to the Electronic Frontier Foundation, intellectual property in the Intertubes is still a vague area, with no set rules, but there are guidlines. According to their FAQ page, here is a rule of thumb for quoting people, posts or articles:
There are no hard and fast rules for fair use (and anyone who tells you that a set number of words or percentage of a work is "fair" is talking about guidelines, not the law). The Copyright Act sets out four factors for courts to look at (17 U.S.C. § 107):
- The purpose and character of the use. Transformative uses are favored over mere copying. Non-commercial uses are also more likely fair.
- The nature of the copyrighted work. Is the original factual in nature or fiction? Published or unpublished? Creative and unpublished works get more protection under copyright, while using factual material is more often fair use.
- The amount and substantiality of the portion used. Copying nearly all of a work, or copying its "heart" is less likely to be fair.
- The effect on the market or potential market. This factor is often held to be the most important in the analysis, and it applies even if the original is given away for free. If you use the copied work in a way that substitutes for the original in the market, it's unlikely to be a fair use; uses that serve a different audience or purpose are more likely fair. Linking to the original may also help to diminish the substitution effect. Note that criticism or parody that has the side effect of reducing a market may be fair because of its transformative character. In other words, if your criticism of a product is so powerful that people stop buying the product, that doesn't count as having an "effect on the market for the work" under copyright law.
Also, in this light, I would like to offer an public apology to Milwaukee Magazine, Bruce Murphy and Erik Gunn, for my own transgressions.