Part of the issue here is defining "market" and their scope. MS has a monopoly on desktops, but how much impact does that have with people increasingly shifting

Part of the issue here is defining "market" and their scope. MS has a monopoly on desktops, but how much impact does that have with people increasingly shifting off the desktop? How do you define Facebook's market? If it's "social media" does that include blogging? twitter? email?