It wasn't feasible to build an effective decentralized social network until a few years ago. It's still not really feasible today but thousands of people are working on it and progress is being made.

I think email is a great example as it's a highly successful protocol that unifies most applications together. When you go to a website you sign up with your email. It's universal. You can guarantee that someone can interface with you via email in a way you can't with centralized services like Facebook. There have been some businesses that primarily work on Facebook but it's uncommon and they always have email as a backup. If you want to notify someone of something or provide an official service it will be over email. Email wins because it's decentralized, warts and all.