Email is technically decentralized. Many of us use hosted mail services, but we could just as easily use our own PC as a mail server.
There are similarly decentralized chat services, such as XMPP.
To quote the homepage: "Matrix is an open standard for decentralised communication, providing simple HTTP APIs and open source reference implementations for securely distributing and persisting JSON over an open federation of servers."
It's not finished by far, since for example there is no end-to-end encryption yet [1], but the specs look really promising.