Probably like email or all the other decentralized services the federated people talk about. Email didn't get mass adoption until centralized servers like Hotmail and Yahoo came online. People don't want to run their own servers and now a days it's a lot more work to do so.
As a "federated person", yes, an email sort of scenario is exactly what I'd like, or at least it would be many times better than what we currently have for im systems. The most important thing to me (and I suspect many other people in favor of decentralization/federation) is the ability to host my own service. I couldn't care less if 90% of email users are on gmail as long as I can run own email server or pay someone else to run one for me while still communicating with everyone.
