Facebook plugin

i have this on error logs [29-Jun-2012 03:27:52] PHP Warning: session_start() [<a href='function.session-start'>function.session-start</a>]: Cannot send session cache limiter - headers already sent (output started at /home/*****/public_html/wp-content/plugins/buddypress/bp-themes/bp-default/_inc/ajax.php:151) in /home/*****/public_html/wp-content/plugins/facebook/includes/facebook-php-sdk/facebook.php on line 39
anyone knows what is wrong?