The fair use defense to copyright infringement was codified for the first time in section 107 of the 1976 Act. Fair use was not a novel proposition in 1976, however, as federal courts had been using a common law form of the doctrine since the 1840s (an English version of fair use appeared much earlier). The Act codified this common law doctrine with little modification. Under section 107, the fair use of a copyrighted work is not copyright infringement, even if such use technically violates section 106. While fair use explicitly applies to use of copyrighted work for criticism, news reporting, teaching, scholarship, or research purposes, the defense is not limited to these areas. The Act gives four factors to be considered to determine whether a particular use is a fair use:
the purpose and character of the use (commercial or educational, transformative or reproductive);
the nature of the copyrighted work (fictional or factual, the degree of creativity);
the amount and substantiality of the portion of the original work used; and
the effect of the use upon the market (or potential market) for the original work.
The Act was later amended to extend the fair use defense to unpublished works.
There is no specific number of words, lines, or notes that may safely be taken without permission. Acknowledging the source of the copyrighted material does not substitute for obtaining permission, and is not used in determining "fair use".
Allowance is made for "fair use" for purposes such as criticism, comment, news reporting, teaching, scholarship, and research. Fair use is a use permitted by copyright statute that might otherwise be infringing. Non-profit, educational or personal use tips the balance in favor of fair use. When you quote a short portion of some news article or blog post - either with or without attribution to the source - especially if you are using it on social media for criticism, comment, or to make a point, this generally falls under the Fair Use doctrine - especially if it does not affect the market for the original work.
On the other hand, plagiarism is when you copy or republish all or a substantial portion of someone's work and attempt to pass it off as if it were your own original work. There is a very big difference between fair use and plagiarism.