It's wildly different from a website company running their public business on another companies infrastructure.
Alright, so then you'd be OK with some phone company, that has a monopoly on a certain geographic area or town, banning anyone who has made a public social media posts that is critical of the company?
Do you see any problems with this position?
But to answer your question, no, I would not be OK with that. Utility companies (in the United States at least) are typically contracted by cities to provide service, and regulated by a public utility commission as well as various laws. I assume denial of service over social media would violate a number of regulations or contractual obligations. And I agree with that.
Now if you're really asking whether or not various service providers should be regulated as utilities and obligated to provide service to everyone, that's a much more difficult (and vague) question that I don't have a easy answer for.