fig generally gets used for development environments since it's relatively easy to install docker, install fig, clone a repo and run `fig up` and get that application running correctly with all of its dependencies including database, background workers, etc. No more of the old "ok, install postgres, install the right gems, etc. just to make some small change in this project" song and dance!

However, it's perfectly suited to run in production for small to medium sized applications. You can even see a sample "Deploy on git push using fig" that I put together here:

https://github.com/nathanleclaire/laraveldocker

It's fun!

How many production environments deploy all containers on a single host?

There's fig2coreos[1] which will translate your fig.yml into systemd files. Then you could deploy across a CoreOS cluster[2]. Pretty interesting.

[1] https://github.com/centurylinklabs/fig2coreos

[2] http://www.centurylinklabs.com/building-complex-apps-for-doc...

A lot? I Guess? Why not?

In fact, I'm amazed at the amount of businesses that don't. Half of what TechCrunch writes about is, in technical terms, Rails CRUD apps. Why would they need more than a single host in the first year?

Because hosts die.

If your not then your missing out on utilising your resources, check out Mesos or Kubernetes for instance.