"... the actual internet is able to do that far more cheaply."
"I imagine the replacement for Facebook will be ... something under the control of its users..."
This is what I am trying (poorly) to articulate. I use a very small proof-of-concept application to accomplish something like this; no website, no third party managing a "service". I paid nothing for this application.
Zuckerberg says, "The reality here is that if you want to build a service that helps connect everyone in the world, then there are a lot of people who can't afford to pay."
Its difficult to understand what he means by "a service that helps connect everyone in the world".
Users already pay for internet access. This internet service is what allows the subscriber to connect to everyone in the world. What he is describing sounds more like a central registry of contact details.
What was the most elusive "starting material" to create a Facebook alternative: the contact and other personal details for most of its users. However this information is, as the world now knows, no longer in the sole possession of Facebook.
For example, 420 million profiles from 2007-2010 were expropriated some 10 years ago. See https://news.ycombinator.com/item?id=16724375
Today in response to media pressure, Facebook announced publicly "most" of their users have likely had their account info expropriated. "We believe most people on Facebook could have had their public profile scraped in this way."
Moreover, with the recent changes Facebook has made, every user now has the means to easily export her data in a portable format.
As a user, I might look up a contact in the "Facebook directory" (of which there might be many copies in many places) and then choose to communicate with her through a different channel, one that Facebook does not control.
This channel does not need to be managed by a third party seeking to profit from selling advertising. It does not have to appear anything like Facebook or any of it subsidiaries, although it could.
Once the contact and I are in touch there is no reason to involve Facebook or any other ad-supported web company going forward.