Next, Blizzard announced that you will be able to appear offline for those times you want to go it solo:
In the coming months, we're planning an update to Battle.net that will give you more control over your online presence when playing Blizzard games. Soon, World of Warcraft, StarCraft II, and Diablo III players will be able to select "Appear Offline" as one of their Battle.net social-status options… When you choose this option as your status, all of your Real ID friends, BattleTag friends, and character-level friends will see you as “Offline” in their friends list whenever you’re logged into a game.
