I'm building a web app which will probably only ever have a few users, but may someday have as many as 100,000 (#wishfulthinking). I need to figure out where to host it. AWS and Azure seem overpowered (and expensive). Heroku is one I've heard of, but don't know much about.

My main criteria are: easy to use; easy to scale; cheap enough to pay for with loose change unless it starts getting a ton of traffic; able to host an SPA and its supporting API with a small database behind it.

What do you use/like?