1) make sure that the "new" version and/or the "whatever" version of facebook that won't load is deleted from your iphone. at this point, you now do not have any app for facebook on your iphone.
2) plug iphone into the computer.
3) click on itunes (will likely sync automatically, just wait until completed).
4) once in itunes you will see 5 major headings on the left hand side (i.e., library, store, devices, genius, and playlists); however, these headings may vary depending on preferences, but i think most will have those five listed.
5) locate "library," and under library is a list of things in your itunes library (i.e., music, movies, tv shows, podcasts, audiobooks, and the most important---apps). this list may vary depending on what is in your library.
6) locate "devices," and under there (if your phone is plugged in) you should see your name and iphone (e.g., mary's iphone).
6) locate "apps" listed under the heading "library."
at this point, your phone is plugged into the computer, itunes is up/open, and you have located "apps" on the left hand side and also see "devices" listed, and under devices, the name of your iphone.
7) click once on "apps" - at this point you will see your apps listed, including your facebook app. locate the facebook app.
8) do **not** 'click' but DRAG the facebook app to your iphone listed under devices. (e.g., drag the facebook app to "mary's iphone").
at that point, facebook should automatically start to load on your iphone. if it doesn't press sync.
i did this and now have 6.6 (older version) of facebook now back on my iphone.
of note: i had deleted my facebook app from my library in itunes (was trying anything, even not so bright things). so, in order to get my original facebook back, i went into my "trash" folder and transferred it back into my app in itunes.
okay, that's all i got. tried to be clear. hope it works out for some of you.