Cable is by all accounts perfectly fine in areas with multiple providers and real competition. Airlines were for the rich in the 70’s. Now they’re flying buses. Unpleasant and cheap. Competition in airlines is so brutal that it’s uncertain whether the US airline industry as a whole has ever turned a profit or if on net it's just burned investor money. In social networks I remember Friendster, Bebo, Google+, MySpace and many others. Snapchat still exists. This is a contestable market and it’s absolutely contested. Search engines likewise. Bing and DuckDuckGo are perfectly serviceable for most purposes, if not academic search, and the switching costs are non-existent.
In industries with little relevant competition it’s either because the government gave one company a monopoly, the Comcast effect, or because they’re just crushingly better than the competition, Alcoa or Google. The dangers of monopoly are in oversold to a completely ludicrous degree compared to the historical record. Look at Standard Oil, the poster child for the trusts. It drove competitors out of business through expertise and economies of scale while increasing quality and decreasing prices. Then the government broke it up after it had started to lose market share anyway. Monopoly gets an insane amount of coverage in economics compared to how often it arises in the real world outside of government mandates[1].
[1]http://png.cdn.mises.org.468elmp01.blackmesh.com/sites/defau...
The Myth of Natural Monopoly
It is a myth that natural monopoly theory was developed first by economists, and then used by legislators to "justify" franchise monopolies. The truth is that the monopolies were created decades before the theory was formalized by intervention-minded economists, who then used the theory as an ex post rationale for government intervention. At the time when the first government franchise monopolies were being granted, the large majority of economists understood that large-scale, capital intensive production did not lead to monopoly, but was an absolutely desirable aspect of the competitive process.