Bad multiplayer etiquette:
Trying to call someone out on a website you know they don't use for all the world to see because you are raging about their style or method of play in a single game, and especially if you don't have your facts straight. Raging at individuals online "behind their backs to the world" is poor sportsmanship. "It's only a game!"
Good multiplayer etiquette:
Contacting them directly 1 on 1 via the appropriate channels to discuss it, and settling your differences mano y mano on the battlefield, instead.
...most people I know would prefer a friendly and competitive 1 vs. 1 challenge over online spamming...