meteor standalone app not working

I built my meteor app, with meteor build --directory, then I did the npm install from the programs/server directory. I set

MONGO_URL=mongodb://<mongo machine URL>:27017/<my db>

and

ROOT_URL=http://localhost:3000

Then, I start my app with node main.js. And the app looks like it starting. But when I try to connect to http://localhost:3000 in the browser, I get nothing. When I try to curl that URL, I get:

curl: (7) couldn't connect to host

I can't figure out what I could be missing.

Any ideas?