If you're running Express 4, it appears to me like you're missing the lines of code:
var app = express(); var server = app.listen(3000);
That will start your web server and set it to port 3000. See the bone simple Express 4 app here: http://expressjs.com/4x/api.html
And, then to start up socket.io, you would add:
var io = require('socket.io').listen(server);
And, there is no need for this line:
var http = require('http');