If the commentator doesn't publicize his or her real name, I'd advise you respect privacy and just give the user name. It may look silly, but it's a research reality. (If you check the Works Cited list at the end of this early article of mine, you'll see I once cited a message board comment from someone whose user name was the very erudite-sounding "PB4Ugo2bed.")

And with the persistence argument...there's always the Wayback Machine. Online content IS being archived, even though it is so malleable.

In MLA writing style, parenthetical documentation enables the writer to acknowledge a quoted source within the text by giving a reference at the exact place where the source of information originated. Readers can later follow up on the fully referenced list on the “Work Cited” page found at the very end of the research paper. In a number of cases, providing a page number and the author’s last name are sufficient. For example Source: http://www.mla-format-works-cited.com/
In response to emerging rapid metropolitan expansion, all urban projects renewal sought "an order where more intellectually stimulating dishonesty, significant conflict, and more complex kinds, may happen" (Mumford 485).

- Amie