The email, as a method of exchanging digital messages is not centralized. Now, it's true that a few providers host most people's email addresses, but there is not a single authority controlling the flow of the email messages.
If the case of a social network, you can't just switch and still remain connected with everyone else, because the provider controls the flow of your interactions. However, you can very easily switch your email provider, or even host your own, and as long as your friends still have the same emails, you're still "connected". I wouldn't call email centralized, but I agree is not decentralized 100 percent.
When you switch providers, you still have to warn your friends about the change. The only way not to is to own your own domain name (whatever that means).
More generally, I think there's an incentive problem here: it is just too convenient to let Big Data host your email. And if you're crazy enough to have a mail server at home, it is just too convenient for everyone else to spam-filter you as a netbot (because you're using a residential IP address).
Even email is centralised these days. Look how many providers host over 90% of people's primary email address.