No, of course not. The service that they sell - access to an audience for advertising - can be bought from many other companies, such as Google, TV networks, the New York Times, etc.
This is the definition of a monopoly from Wikipedia:
"A monopoly exists when a specific person or enterprise is the only supplier of a particular commodity. ... Monopolies are thus characterized by a lack of economic competition to produce the good or service, a lack of viable substitute goods, and the possibility of a high monopoly price well above the seller's marginal cost that leads to a high monopoly profit."[1]
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[ 150 ms ] story [ 83.9 ms ] thread"A monopoly exists when a specific person or enterprise is the only supplier of a particular commodity. ... Monopolies are thus characterized by a lack of economic competition to produce the good or service, a lack of viable substitute goods, and the possibility of a high monopoly price well above the seller's marginal cost that leads to a high monopoly profit."[1]
What's your definition?
[1] https://en.wikipedia.org/wiki/Monopoly