Why isn't concat-stream working with process.stdin.pipe()?

Here's my code:

var concat = require('concat-stream');
process.stdin.pipe(concat(function(){console.log("output")}));

What I'm expecting this to do is output "output" every time I enter input into the console, but this doesn't work. Does anyone have an idea why this isn't working? If I do a fs.createReadStream() buffer, it works fine, just not with process.stdin.pipe(). I've used process.stdin.pipe() for other things though, and they worked fine.

Thanks in advance!