Companies growing does not make them into utilities. Utilities are providers of very specific commodity services which are specifically defined by statutory law.

Monopolies (as in anti-trust law) are companies which abuse their power to hurt the consumer. Traditional anti-trust law doesn't work against social media companies because consumers pay no cash for the transactions. We could change anti-trust law, but since there is no analog, it's not clear what we would change it to.

> There certainly is precedent for governments compelling utility providers to not restrict their services arbitrarily.

There is also precedent for governments to uphold a concept of "decency" (the same government that defines it as "I know it when I see it") which communities can judge for themselves, without a written definition. I, personally, don't see the judgements that social media companies make as "arbitrary" (they do have written ToS and they attempt to give their content moderators guidelines/baselines for judging decisions).