Social networks as companies are both centralized and decentralized. They use 21st century technology that allows ‘decentralized value’ creation (notice the comillas). People creates contents for others in the form of pics, posts and videos. If you want to access those contents you need to sign in FB.
But economically FB works in the 17th century. Its capital divided into shares, needs to generate yield for its shareholders, and that’s the legal purpose of its existence.
This way, part of the value created by millions of users is extracted and converted into capital return for a small group of shareholders. The mechanics to do it is as simple as advertising: selling the attention of the users to the advertisers.
And the rest of the value is shared among the users, so they are constantly updated about what other users are doing or saying. All of this creates network effects and economies of scale very difficult or impossible to replicate unless you switch to a different economical paradigm.