I had FB on my iPod Touch and iTunes. When I got the iPhone, it was already preinstalled. For some reason, there are two different versions and I now have both of them on my iPhone. One is a blank page and the other one works. When I try to delete the blank, it messes up both. So, try installing it again. I have a feeling my issue is that one is an actual app and the other may just be the FB page I "bookmarked" from Safari.
Oh, and I just leave both on my iPhone. The blank one sits on it's own homescreen by itself and I never use it.