I was having the same issues on my HTC Thunderbolt. I was able to get rid of mine by going into my apps and finding the App called Downloads. It showed that the Facebook (build 140579) was In Progress of downloading. So I killed it to see if this would work. I restarted my phone and it still hasn't popped up. I checked to see if it was in Downloads, and there is nothing there.
Prior to figuring that out I had originally tried to force stop the Download Manager. That worked, however when I restarted my phone it would pop right back up and say that it was downloading again.