All Android phones have been seeing this issue this week. Hero, Moment, Evo, etc. I don't know what's wrong, but I can guess that Facebook may not be fully available.
I was having this problem as well on my Hero and this is how I fixed it:
Hope this helps. I found these suggested steps on the HTC Sense Facebook discussion boards. Apparently, like Will says, it's happening to a lot of android phones right now.