Social media would certainly have happened without capitalism - for example, Mark Zuckerberg originally created Facebook to solve a problem that he personally had, and network effects guarantee that a single service would eventually win. It’s just that without capitalism, that’s where the story ends. Companies like Facebook and Google would look very different (and more ethical) without the relentless drive for horizontal and vertical integration - which would be illegal if the US enforced existing antitrust law, anyway.
It’s not about the users at all, it’s because moderation is labor intensive and Facebook doesn’t want to pay for it. That’s why tech companies worked so hard to get safe harbour laws, and why they’re so reluctant to engage in moderation at all. Facebook doesn’t want to make users/regulators aware that they do have the means to moderate their service, and that when malicious or illegal content stays up it is because they are deliberately ignoring it.
What you see is what you get. ARPANET was decentralized in the sense of routing; initially the vision was to have a communication mesh too dense for an adversary to feasibly disrupt, rather than having single points of failure. Doing this is still theoretically possible. Services, however, were never decentralized. Internet addresses, after all, do not describe services, they describe the physical location of the recipient. And every one of the earliest internet services, while sometimes designed for redundancy, always require you to know the location of some proximate and trustworthy endpoint.
There is one: e-mail. The problem with e-mail is that you can’t discover, follow, and broadcast - and apparently these are important features to some stupid crazies.