function extractParamFromUri(uri, paramName){
    if(!uri){
        return;
    }
    var uri = uri.split('#')[0]; //remove anchor
    var parts = uri.split('?'); //check for query params
    if(parts.length == 1){
        return; //no params
    }

    var query = decodeURI(parts[1]);

    //find the url param
    paramName += '=';
    var params = query.split('&');
    for(var i = 0, param; param = params[i]; ++i){
        if(param.indexOf(paramName) === 0){
            return unescape(param.split('=')[1]);
        }
    }
}

twttr.events.bind('tweet', function(event) {
    if (event) {
        var targetUrl;
        if (event.target && event.target.nodeName == 'IFRAME') {
            targetUrl = extractParamFromUri(event.target.src, 'url');
        }
        _gaq.push(['_trackSocial', 'Twitter', 'tweet', targetUrl]);
    }
});