> The only plausible answer is that they have a dominant market position

Not necessarily. If the internet were as essential to average daily life today in the US as electricity (which I am not declaring true or false in this particular sentence), then it would be reasonable to treat ISPs but not websites as common carriers no matter how competitive the internet access market is.

Another answer that doesn't rely on the previous one would be that it is not reasonable to expect a social media site to deliver a user's speech as reliably as one would expect an ISP to deliver the speech. I got this idea from an excerpt on Wikipedia [1]:

> Cases have also established limitations to the common carrier designation. In a case concerning a hot air balloon, Grotheer v. Escape Adventures, Inc., the court affirmed a hot air balloon was not a common carrier, holding the key inquiry in determining whether or not a transporter can be classified as a common carrier is whether passengers expect the transportation to be safe because the operator is reasonably capable of controlling the risk of injury.

If you're referring to common carriers offering transport of physical goods or people (as cargo railroads and passenger railroads do) then a different answer could be that users can sometimes have priority over physical property owned by someone else, while treating websites as common carriers would mean giving users priority over physical property owned by someone else which the owner also uses (e.g. puts terms of service on and moderates) to express what kind of website the website is. In other words, outweighing property rights alone is easier than outweighing property rights and speech rights combined.

Moreover, I'm guessing that whether the public associates an ISP with third-party speech depends on whether the speech is protected. Suppose that a social media site is notorious for users who speak in favor of copyright infringement (copyright infringement being unprotected speech). The public might associate the ISPs those users use with copyright infringement (anecdote: Reddit, Frontier, and copyright infringement [2]). But what about protected speech? A social media site is notorious for users who speak in favor of hate crimes. Will the public be comparably likely to associate the ISPs those users use with hate speech?

But nothing I wrote above is not really an answer, since I've been speculating for the most part. I'm not very knowledgeable about the legal theory behind a common carrier designation, so here is something closer to an answer from Mike Masnick [3]:

> As you look over time, you’ll notice a few important common traits in all historical common carriers:

> 1. Delivering something (people, cargo, data) from point A to point B

> 2. Offering a commoditized service (often involving a natural monopoly provider)

> In some ways, point (2) is a function of point (1). The delivery from point A to point B is the key point here. Railroads, telegraphs, telephone systems are all in that simple business — taking people, cargo, data (voice) from point A to point B — and then having no further ongoing relationship with you.

> That’s just not the case for social media. Social media, from the very beginning, was about hosting content that you put up. It’s not transient, it’s perpetual. That, alone, makes a huge difference, especially with regards to the 1st Amendment’s freedom of association. It’s one thing to say you have to transmit someone’s speech from here to there and then have no more to do with it, but it’s something else entirely to say “you must host this person’s speech forever.”

> Second, social media is, in no way, a commodified service. Facebook is a very different service from Twitter, as it is from YouTube, as it is from TikTok, as it is from Reddit. They’re not interchangeable, nor are they natural monopolies, in which massive capital outlays are required upfront to build redundant architecture. New social networks can be set up without having to install massive infrastructure, and they can be extremely differentiated from every other social network. That’s not true of traditional common carriers. Getting from New York to Boston by train is getting from New York to Boston by train.

[1] https://en.wikipedia.org/wiki/Common_carrier#General

[2] https://arstechnica.com/tech-policy/2024/02/reddit-beats-fil...

[3] https://www.techdirt.com/2022/02/25/why-it-makes-no-sense-to...