All courts do a case-by-case examination of the facts, using a “Four Factor test” to analyze whether the doctrine Fair Use applies in a given situation.
The four factors are stated in the opinion of the Joseph Story in Folsom v. Marsh, 9 F.Cas. 342 (1841) (widely cited – even in the most recent of cases). This case had to do with a guy (the defendant) who'd copied hundreds of pages from the plaintiff’s multiple-volume biography of George Washington, so as to produce a separate work of his own. (Most of what he copied are letters Washington wrote – so there is also the public right need to know analysis – but much of this was not original.)
If you want a book on the subject that cites a number of sources and discusses all of the law on copyright and fair use, Infringement Nation: Copyright 2.0 and You, by John Tehranian, is a good resource for people in your position as it cites many cases and scholarly articles as well as discussing the landscape of using copyrighted works in today's society.
Under this case, what became abundantly clear is that Fair Use is not a finite legal rule with a one-size-fits-all definition. It is also just an affirmative defense if you're ever sued under the theory of copyright infringement. To know if your site is a fair use, I'd objectively try to apply the test a court would. It sounds to me like you'd be fine, but only you know the intricacies of your site.
Folsom is often touted as the landmark case that created safeguards of copyright law against over-protection and for maintaining the balance between private exclusion power and public "accessibility to information and knowledge."
Notwithstanding the provisions of...copyright laws....the fair use of
a copyrighted work, including such use by reproduction...for purposes
such as criticism, comment, news reporting, teaching ...is not
an infringement of copyright.
In determining whether the use made of a work in any particular case is a fair use the factors to be considered (now codified in 17 U.S. Code § 107) shall include:
The purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
the nature of the copyrighted work;
the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
the effect of the use upon the potential market for or value of the copyrighted work.