Well, sure, 100% national market share is obviously a monopoly. But you're also a monopoly if you distort a market (generally markets with less than 3 major players) or have control or distortion over individual markets (you can have a monopoly in NYC but not the US, for example).
I think this obviously applies to telecoms, which often have monopolies over certain areas (and often granted by local government! thanks a lot Philly city council).
I think there's a whole host of bad behavior that's non-monopolistic. That's what I think needs to be cracked down on. Some of these do relate to market share, but I don't think it's as simple as a boolean monopoly / not-monopoly. It's a gradient, and much of the behavior in that gradient should be dealt with. There's often collusion between companies on pricing, or agreements not to compete, etc...
