Fatal error: Cannot use object of type stdClass as array in /sites/all/modules/fb/fb_app.module on line 340

From the README.txt: The Facebook client libraries has bugs if you do not have the JSON extension
for PHP installed, see http://us.php.net/manual/en/json.requirements.php.

See also http://bugs.developers.facebook.com/show_bug.cgi?id=4351

agree , facebook client libs suck , big problem for none-developers (like me) that does not have idea on how to intall the JSON extension

:(

....
Just found some instruccions here
http://slaptijack.com/system-administration/lets-install-json-for-php-5/

Not sure about whether this issue is still alive, but I stumbled upon it when installing beta9, and found easier to code the attached hack than fighting with json libs. Anyway, I'm still not sure the problem is present in other parts of the module, but the solution would be the same.