No, there is no cloud service offered. No one is providing Realm as a hosted, managed service. That said, you can get VM images prepared with Realm. One example is on Azure Marketplace.
You basically need to host the database on several VMs yourself. Having Realm as a service would be great -- but at the same time -- hosting it yourself means you are not locked in to a single cloud vendor.
I would not be surprised if Google, Microsoft and Amazon start to offer Realm-like technology, just like they once all started offering NoSQL-databases.