Reference JSON Object Using String Path - JavaScript

Question

How can I get the value of json object using a series of keys stored in a string. So in this object :

var data = {
 "user":{
    "profiles":{
       "facebook":{
          "id":765764
       }
    }
 }
}

I can reference the facebook object this way :

data.user.profiles.facebook; // = Object{id: 765764}

but what if I have the keys saved in a string like this :

var key = "user.profiles.facebook";
data[key]; // does not work unless the string contains only one key.

I need to reference the object where I can modify/change it.

data[key]= {};// for example

How can I do it ?