Reich writes:

"But where else would consumers go to post short messages that can reach millions of people other than Twitter?"

Notice the problem: he has defined the market very narrowly. The market he is claiming Twitter has a monopoly in is the market for "post[ing] short messages that can reach millions of people." In that narrow market, Twitter does dominate because that is Twitter's niche; that is how they chose to compete against other firms.* If Reich used a broader definition of the market, then Twitter's supposed platform monopoly no longer holds.

One of the major issues that arises with antitrust is "what is the relevant market?" The more narrow the definition, the more likely a firm can be considered a monopoly. The broader the definition, the less likely.

This matter of definition proves a real problem for antitrust enforcement. If the regulators rely on too narrow of a definition, then they will squash the very competition they are trying to promote. If they rely on too broad a definition, they may miss genuinely harmful instances of market power.

Time complicates the analysis further. The 2nd Law of Demand states that the longer a price remains relatively high, people will search out more substitutes. Indeed, the demand (and supply) of goods generally become more elastic over time. Thus, a narrow definition may be appropriate in the near term but in the long term it is not.

There is a way around this definitional problem: focus on reducing barriers to entry rather than breaking up firms. Create the conditions for competition to flourish. Any economic condition necessarily has a shelf-life. Consequently, interventionist policy (even ignoring public choice and Coasian institutionalism issues) naturally have a half-life. The costs of regulation are likely higher and the benefits likely lower than economists estimate. So, focus on achieving the same goal but via a more cost-effective and flexible approach: identify the institutions at play and focus on those to generate more competition rather than trying to navigate the complex and ever adapting/changing economic landscape.

*It's worth noting, however, that consumers have many other options to post short messages that can reach millions of people including (but not limited to): Fb, LinkedIn, Substack, Wordpress, IRL bulletin boards, billboards, newspapers, etc. So, Reich is incorrect even given his narrow definition.

This analysis holds even for supposedly natural monopolies.