removing facebook removes mp-harmattan-005-pr, which is the meta-package that most of harmattan rests upon. it is FINE to remove this, if you know what youre doing; just dont autoremove all the necessary packages, instead mark them as manually installed. and use apt-cache depends to mark the dependencies as manually installed.
~ $ apt-cache rdepends facebook facebook Reverse Depends: mp-harmattan-001-pr mp-harmattan-001-pr ~ $ apt-cache depends mp-harmattan-001-pr | grep -i facebook Depends: account-plugin-facebook Depends: webupload-service-facebook Depends: facebook Depends: signon-facebookplugin Depends: l10n-app-facebookshare Depends: l10n-app-facebook