The spirit of "fair usage" is so that an original work can be critically reviewed, or as part of a research/academic endeavour that builds on the previous work, therefore allowing the reproduction of portions as a part of the process.
It is not meant for copying material to "stand alone". A small number next to a quoted peice of text linking to the original would not be enough in our view. You have to make it very clear that what you have quoted is somenone elses work and not yours, and attribute it.
In an academic or research work, readers are used to the still of numbers relating to the priginal work, but almost always the name of the original source or author is also referred to in the text.
you need to ask yourself whether the method you use for attributing content is "fair use". Ask yourself your motive - am I reprinting this material just to add content to a page so that it almost looks like I did it, or an i reprinting it as part of a review, and/or adding value where of course the orginal authorship is a key part.
For our part as a site with much commissioned original content, we would not be happy if people reproduced more than a paragraph without making it clear that the work was ours. A small number, in our view would not be enough, and we would be suspicious that we were being quoted as a way to fill up another page, rather than in the context of critical/satirical review and in the spirit of adding value to the work.
That's the spirit of the law. How it is interpreted is of course the job of the lawyers. It is also the traditional interpretation. Internet publishing adds some new elements to the equation, including the value of keyword optimised links, and maybe even the sophistication of a site's readership to be able to understand clearly that the quoted portions of an original work belongs to someone else, and that the there are clear pointers to the origonal and how to access it.