I have had the same issues, I think it is often the bot's problem

Just to be certain check your links are correct and manually test them. Also ensure your sitemap is up to date and that you are not blocking the crawlers with metarobots, robots.txt, or some weird stuff in htaccess.

I have found that renaming pages or moving them will often cause 404 issues with crawlers

Just use an SEO plugin and canonicalize allt he archives and categories, don't use no-follow as it is a will suck away your pagerank. If you must you could also use the metaname robots and set it to follow, noindex

btw I think Pashmina should win an award for cutest SEO on seomoz

Thanks for the input guys. I have taken your suggestions on board and made some additional optimizations and edits as well. I will let you know how it goes.

Ryan does (harshly) make some decent points although I am already very familar with all the stuff listed in the seomoz seo guide....

No onsite changes were made in the last few months at all and no blackhat linkbuilding has been done.

Inspired (and annoyed - haha) by Ryan response I decided i would actually go through and do an onsite audit of the page and did find some disturbing things...

The biggest was that large chunks of the content appear on multiple other websites. I am also concerned that the actual content may be seen as having overused the term 'facebook' and also the xml sitemap was broken..

Do you guys think these factors could be related to the sudden drop for this page (no other pages on the domain are having issues)

updated with url and kw

the site still ranks for its domain name and ranks in the top 10 for other queries - it is just this one keyword that the rank has dropped for - unfortunately it is the websites money keyword

A page of mine  (http://loginhelper.com/networks/facebook-login/) was ranking in the top 10 for keyword (facebook login) and has been for at least 2 months, moving between 5th and 10th.

Suddenly in the last 3 days the rank for the keyword dropped from 7th to 46th, yet none of the other keywords have been affected (they target other pages) and their ranks have continued to improve.

I am trying to figure out what caused this sudden drop in the ranking of 1 page (the page has quality mainly text based content and isn't in the least bit shallow or spammy)

I have been thinking perhaps a crawl or server error may be to cause leaving the page temporarily unavailable or with a big load time... Otherwise what could cause one page to drop so much so quickly whilst other pages improved their rank?