Lumping in the phone companies with social media companies is fallacious. The phone companies and ISPs control a finite resource in the physical environment to the exclusion of other companies. Websites are not like this, they are decentralized nodes on a virtual network, the popularity of one node does not impact the capabilities or accessibility of other nodes. Arbitrary standards of popularity aren't a good metric for applying regulations, individual users make their own choices to visit various sites based on their own capricious preferences, nothing is forcing an individual to use a particular site except for their own perceptions on what they site might have to offer them.
Right, that's the obvious argument against gov't regulations, that applies to all monopolies and is exactly what you'd expect their lawyers will say. My counter to your argument is that Twitter is "effectively" a monopoly, even though smaller competitors exist.
Any monopoly can claim that since were all in a free market system, it's merely customers making free choices that's causing an incorrect "appearance" of a monopoly to exist. But you can ask any journalist, legislator, or other public figure if there's another 'competitor' that can provide what Twitter provides, and the answer is simply "no".
If we were talking about gaming consoles or some other type company the monopoly perhaps wouldn't matter, but we're talking about communications infrastructure for the entire nation, and frankly world. We simply cannot let a handful of unelected overlords censor the world.
Someone is always going to be the biggest, you can't just label something popular as a monopoly and then say "the smaller competitors don't count", of course they count, the entire problem with a monopoly is that competitors can't survive because the monopoly has exclusive control of the market, but this isn't the case with websties, if my site has 10 million users it isn't in any way impacted by the fact that another site has 100m users.
