I can't say that he was or was not a Fallen Lord, because different people within the game seem to disagree on whether he was or not, and I think the disagreement stems from using different definitions of "Fallen Lord"--either "one of 6 evil sorcerers" or "one of 6 evil sorcerers + the guy who led them"--rather than from factual arguments about the nature of Balor. So the answer depends on your definition.
Nut if you're interested in this because you want to know whether Balor was one of the six powerful sorcerers, then no, he was not. The two Lords we don't see in the game (from pregame info--see http://myth.bungie.org/legends/delusions/names.shtml) were known as the Voiceless One and the Faceless Man, neither of which really fits Balor. And the other four Lords are already accounted for. So it seems that Balor would be counted as a seventh.
--SiliconDream
