Broadcast message to all clients

You are calling the on function on the wrong object. You should rename the socket variable to something else. Most commonly, people use io.

io = require('socket.io').listen(server);

Then, you will need to call emit on sockets to send to all users:

io.sockets.on('connection', function(socket) {
    io.sockets.emit('message', data);
});

Of course, send is an emit that uses message as the event. So you can use it instead inside your callback:

io.sockets.send(data);

In either case, you then have to do the following in your client to react to this event:

socket.on('message', function (data) {
  // Do things with data
});

Note: socket.broadcast.emit can be used to send an event to all users connected other than the user that just connected as the socket.