Here's an interesting resource on the "Four-Pronged Fair Use Test". Evidently there's no clear "this is fair use" vs. "this is plagiarism" litmus test, but instead there are four factors that are taken into consideration, with sort of a sliding scale on each of the four factors. This webpage is good reading, but for a real quick and dirty summary, the four factors are:

 (1) What is the character of the use? (commercial, commentary/criticism, educational, nonprofit, etc.)

 (2) What is the nature of the work to be used? (factual article, fictional story, or a combination, with using somebody's factual reporting more likely to be considered fair use, whereas copying someone's fictional story to be more likely to be considered plagiarism)

 (3) How much of the work will be used? (all of it, or just parts)

 (4) What effect would this use have on the market for the original or for permissions if the use were widespread? (is quoting something going to mean that whoever wrote it makes less money or loses control of their own work?)

So, in terms of Memestreams (and I have to preface this with IANAL: I Am Not A Lawyer):

Mostly good things to do:
- Quote a small amount of text rather than an entire article
- Quote factual articles
- Add your own commentary or thoughts on what you quote
- Always link back or attribute the source

Sometimes bad things to do:
- Quoting large amounts of a webpage
- Quoting someone's original fictional story
- Quoting from inside a "subscription only" site
- Saying words are yours, when instead they're copy/pasted from somewhere else
- Posting just a copy/paste, without any kind of added original commentary

And again, none of the above items are an instant "good" or "bad" determination. They're just recommended guidelines, that are probably worth mentioning in a Memestreams help file somewhere. :)

My $0.02,

Elonka :)