The browser is telling you, "hey, facebook needs to take a look at your system so you can tell it where to get the photos from. And oh, by the way, the digital certificate has been verified and deemed safe by a third party."
All the certificate tells you is that it's really from Facebook, not that the software is safe. You have to trust Facebook to be sure of that.
That said, I agree that there's nothing to worry about with the Facebook app. I'm sure all it does is upload photos.