Async loading of javascript with callback

gistfile1.js

(function(d, s) {
var js, fjs = d.getElementsByTagName(s)[0], load = function(url, id, callback) {
	if (d.getElementById(id)) {return;}
	js = d.createElement(s);js.src = url;js.id = id;js.async=true;
	fjs.parentNode.insertBefore(js,fjs);
	if(callback!=undefined) {
		js.onload=callback;
		//Extra for internet explorer
		js.onreadystatechange = function () { if(this.readyState == 'complete' || this.readyState == 'loaded') { callback(); } }
	}
};

load('//connect.facebook.net/en_US/all.js#xfbml=1', 'fbjssdk', function(){_ga.trackFacebook()});
load('https://apis.google.com/js/plusone.js', 'gplus1js');
load('//platform.twitter.com/widgets.js', 'tweetjs', function(){_ga.trackTwitter()});
}(document, 'script'));