I'm having the same issue as you. I'm using the iframe version of the activity plugin , and it seems that I'm obtaining the same response.
My research pointed me to the facebook fan page permissions, may you please check the permissions of the fan page (or group page).
Here's the link that I found.
Facebook Social Plugin Not Working
If that is not the case, the other work around that I'm working, is to create a very simple facebook web application, and using the new javascript api, I will just pull the posts from the graph. The downside of this, is that the user will need to log in, though, I don't know if there's an easy way out to work with the graph without login, since it beats the purpose of using the plugin that doesn't needs the log in.
<html>
<head>
<title>Activity Feed Test</title>
<script type="text/javascript" src="http://connect.facebook.net/en_US/all.js"></script>
</head>
<body>
<div id="fb-root"></div>
<script type="text/javascript">
FB.init({
appId: here-you-put-your-app-id,
status: true,
cookie: true,
oauth: true
});
FB.login(function(response){
FB.api('/your-fan-page-facebook-entity-here/posts', {limit: 20}, function(response) {
console.log(response);
});
});
</script>
</body>
</html>
Hope that it helps. (In the console log, you will find the response from the activity, I just need the posts )
