PostTo callback isn't called whatsoever.
Look at this sample app (based on a sample by MySpace Developers Team).
I put this on the Canvas page but the callback is never called.
Go ahead and try this yourselves.
Any ideas why? 
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<script>
window.osContainer = opensocial.Container.get();
function postBlogCallback(){
var os_token = MyOpenSpace.MySpaceContainer.OSToken;
var message = opensocial.newMessage("I am a blog post");
message.setField(opensocial.Message.Field.TITLE, "About Testing");
message.setField(opensocial.Message.Field.TYPE, MyOpenSpace.PostTo.Targets.BLOG);
osContainer.postTo(os_token, message, null, ptCallback);
}
function ptCallback(someVal){
if (someVal == MyOpenSpace.PostTo.Result.ERROR) {
document.write('callback gave error')
}
else if (someVal == MyOpenSpace.PostTo.Result.CANCELLED) {
document.write('user cancelled PostTo')
}
else if (someVal == MyOpenSpace.PostTo.Result.SUCCESS) {
document.write('WE ARE MEETING WITH GREAT SUCCESS!!!')
}
else {
document.write('Unrecognized response: ' + someVal)
}
}
postBlogCallback();
</script>