I have this exact same problem, have been using facebook chat since day 1 on my iphone 2g with no prblems at all (apart from the odd glitch here and there) but in this last week my facebook app constantly fails to vibrate or even display messages on chat from friends, Ive tried deleting the app from both Itunes and my phone and re downloading/installing numerous times without any luck.
It really is annoying to say the least as people think im ignoring them! Any ideas?
thanks in advance.