Using sinon and mocha to test node.js http.get

Let's say I have the following function

'use strict';
var http = require('http');

var getLikes = function(graphId, callback) {
    // request to get the # of likes
    var req = http.get('http://ift.tt/rrPc6B' + graphId, function(response) {
        var str = '';
        // while data is incoming, concatenate it
        response.on('data', function (chunk) {
            str += chunk;
        });
        // data is fully recieved, and now parsable
        response.on('end', function () {
            var likes = JSON.parse(str).likes;
            var data = {
                _id: 'likes',
                value: likes
            };
            callback(null, data);
        });
    }).on('error', function(err) {
        callback(err, null);
    });
};

module.exports = getLikes;

I would like to test it with mocha AND sinon, but I don't get how to stub the http.get.

For now I'm doing a real http.get to facebook, but I would like to avoid it.

Here is my current test:

'use strict';
/*jshint expr: true*/
var should = require('chai').should(),
    getLikes = require('getLikes');

describe('getLikes', function() {

    it('shoud return likes', function(done) {
        getLikes(function(err, likes) {
            should.not.exist(err);
            likes._id.should.equal('likes');
            likes.value.should.exist();
            done();
        });
    });

});

How can I achieve what I want, without relying on something else than sinon? (I don't want to use the request module to perform the get, or using another testing lib)

Thanks!