Why is my facebook icon on the ipad dark grey?

Try also settings> general> storage and icloud usage> storage> manage storage> wait for the list of apps to open> delete Facebook from there. Then re-install it.

In the meantime you can always still go to Facebook.com on Safari.

It is stuck as if it was pending a update. Check app store to see if there is an update. If not, try to sync it via itunes to force either an update, or syncing a previous version stored in itunes.