Datadog and SignalFX are both far more scalable, easier to use, and have more features than Prometheus and are SaaS offerings. Prometheus is about 5 years behind them in terms of engineering effort, I believe.

Not yet, but one or more companies are working on this as part of their offerings in the future. Stay tuned!

Note that a hosted Prometheus service has different design tradeoffs. As Brian mentioned, Prometheus as it is is really meant to be run as close as possible to your monitored services for maximum reliability. Also, there's no clustered and long-term durable storage for a similar reason, which will likely be different in hosted versions.

There's noone presently offering that, it's designed more with a view to being on-prem for reliability (http://www.robustperception.io/monitoring-without-consensus/).