я делаю так

Код AS3:
var data:Object = {};
var picURL:String = "some picture absolute URL.jpg";
var name:String = "Some text";
var caption:String = "Some text";
data = JSON.decode('{"message": "Some text","picture": "'+picURL+'","name":"'+name+'","caption": "'+caption+'","link":"lonk to your app"}');
Facebook.ui('stream.publish', data, publishResult_handler, "popup");