I don't think those are actually natural monopolies. But there are ways to compete when you are small and you can't compete on the usuals (price, distribution, etc). It takes creativity though.
I remember when the local telecom monopoly in my town held ski-high Internet prices, regular people started laying up Ethernet cables in the air between high-rises. A new ISP was born that way, who now has completely wiped out the original telecom.
Of course though, there is now a regulation against doing that kind of thing...
