Facebook is one of the most popular iOS apps of all time, so when its crashy, buggy, not downloading, or otherwise causing grief to even a small percentage of the people using it, we hear about it. This time it's with version 6.7.1, released on November 8, which won't download and keeps spitting out iTunes errors if you try. It's a bug fix update, which hopefully would have fixed previous problems with crashes, but it's impossible to tell right now.
The good news is that Facebook gets pretty quick turn-around time from Apple reviewers, so if it isn't working for you now, hopefully it will be soon. If you are having problems with Facebook on your iPhone or iPad, however, let me know with which version - if you can tell or can check your App Store updates tab - about it in the comments.