The web is decentralized. That's the entire point of it. You don't need permission from anybody to start serving web pages.
Don't take this as talking down cryptocurrencies. It's that Bitcoin is as decentralized as the web that is the key innovation. That anyone can build applications on top of it without asking for permission is unheard of in the financial space.
There are many decentralized permissionless systems including e-mail for messaging and IPFS for distributed storage. They do not generally need tokens or proof-of-work to function.
(Sorry for substituting Ethereum above. Ethereum isn't really a good example of a decentralized application as it is governed in full by a Foundation which have all developers on payroll. It is not a multiple equal stakeholder project yet.)
As to why people are talking about Facebook on Ethereum, I don't think the article suggested that. It merely offered Facebook as a generall ballpark to which scale a popular application can reach.
