//replace bbpress with discussion on activities
add_action( 'bp_get_activity_css_first_class' , 'update_bbpress_activity_with_discussion', 10 , 2 );
function update_bbpress_activity_with_discussion( $text ) {
 	if($text =='bbpress')  {
	 $text = 'discussion';
     }
	return $text;
}