The only way to show the feed dialog in the same window instead of popup is to use the direct url redirection.
You can use it like this-
function facebookInit() {
var app_id = "{app-id}";
var fb_description = "{description}";
var redirect_uri = "{redirect-url}";
var link = "{link to share}";
var name = "{name}";
var url = "https://www.facebook.com/dialog/feed?"+
"app_id="+app_id+
"&caption="+fb_description+
"&link="+link+
"&name="+name+
"&redirect_uri="+redirect_uri;
location.href = url;
}
Edit:
The documentation says-
If you are using the JavaScript SDK, this will default to a modal iframe type for people logged into your app or async when using within a game on Facebook.com, and a popup window for everyone else
If your app is inside the facebook, then only it will open as a model iframe type, else, a popup or completely on window will be shown.