Citron (who founded Discord) also was involved in OpenFeint, a mobile game social SDK that started out originally as a game for the iPhone called Aurora Feint. So it's kind of cyclical in a fun way - game, social tool, game, social tool.

I expect this is a pretty common story on a wider basis and people sometimes overlook it. How many bug trackers or build systems started out as internal tools for managing some other product? Etc.