This, but also, they run a centralized system for intellectual property reasons. Facebook Inc. offers a product, called Facebook, where you post content and make connections with other nodes in a large graph. It was built on prior art, but arranging and packaging them together in their particular way was novel.
You could wholesale copy Facebook's public APIs and implement your own "Facebook-compatible clone", in the sense that a third-party unofficial Facebook client could connect to you instead and operate 100% identically. Of course, you can't actually do this, as you'll be threatened with legal action.
Gmail, another frequent example of decentralization overcome by centralization, isn't nearly as egregious in its domain. Gmail is simply an Email-as-a-Service with a bundled webmail client, albeit with a very large market share of email traffic. You can get (most of) your data out of Gmail, and use another email client instead. But Facebook operates over the "Facebook protocol", and no one else can do so. These proprietary protocols are the real threat, not 'centralization'.
>Gmail, another frequent example of decentralization overcome by centralization
How did Gmail do that though? I suspect it largely had something to do with ISPs blocking port 25 for anyone except expensive "business class" users under the auspices of stopping spam. Well, spam never stopped, but about 80% of email in the US runs through 10 domains now. Further "stopping spam" measures meant that those big 10 basically block email from small servers by default and anyone wishing to send email to them has to jump through numerous hoops, further solidifying this centralization.
Centralization starts at the ISP. It starts with asymmetric bandwidth where you get 4x to 10x max download speed vs upload. It starts with port blocking. Centralization is monopoly. As you pointed out, nobody can compete with these large companies because of "intellectual property" laws. If there will be change, it has to start with legislation. You can't code around this unless you have billions of dollars in patents to fight with.
You could wholesale copy Facebook's public APIs and implement your own "Facebook-compatible clone", in the sense that a third-party unofficial Facebook client could connect to you instead and operate 100% identically. Of course, you can't actually do this, as you'll be threatened with legal action.
Gmail, another frequent example of decentralization overcome by centralization, isn't nearly as egregious in its domain. Gmail is simply an Email-as-a-Service with a bundled webmail client, albeit with a very large market share of email traffic. You can get (most of) your data out of Gmail, and use another email client instead. But Facebook operates over the "Facebook protocol", and no one else can do so. These proprietary protocols are the real threat, not 'centralization'.