Indeed, and so can I. However, there is a huge crowd outside of HN that is neither capable of running their own servers, nor are they remotely interested in learning to do so.
Also, I would argue that in practice email is very much centralized, with a few large providers that manage all the complexity for their users (such as spam blocking and maintaining servers).
The point is that social networking could become a decentralized protocol similar to email. Imagine browsing something that looks just like Facebook but underneath each friend's profile it would say "Hosted by Facebook" or "Hosted by Microsoft" or "Hosted by [Startup]". It's feasible technically although Facebook's closed network effect is extremely strong.
As long as our legal structure allows walled gardens and enforced incompatibility, that's what we'll get.
We need to look at websites as service providers/data carriers akin to telcos. The government has issued a slew of regulations to prevent artificially strong network lock-in with telcos, mandating wireless number portability and certain levels of device compatibility.
We need to recognize that Facebook is not a content creator as such, but a neutral carrier who transmits the data generated by its users, and that the users need their freedom and mobility to be respected if we're going to have a free and competitive market in cyberspace. There is no good reason that Facebook or Google should have such excessive unnatural ownership and access blockage rights.
n.b.: I say this as someone who is very conservative politically. Intellectual property is a government-granted monopoly that we have let run far afield.
I don't resent property ownership in the slightest, nor do I resent profit-making. I do resent an oligopoly exploiting regulatory capture and public technical ignorance (cf. Clarke's Law) to pass laws that allow them to arbitrarily and brutally crush innovative and competitive entrepreneurs.
The government is involved. The CFAA and similar laws allow Facebook et al to crush anyone who attempts to break their stranglehold. That's exactly the problem. Alternatives to Facebook that may have otherwise been wildly successful were sued out of existence this way.
The government has given Facebook offensive weapons against competitors. They either need to respond by giving the consumer defensive weapons against corporations whose zeal for intellectual property seeks to block or subsume the consumer's access to the marketplace, or they need to remove their interference all together.
Also, I would argue that in practice email is very much centralized, with a few large providers that manage all the complexity for their users (such as spam blocking and maintaining servers).