function ray_bp_page_title( $title ) {
	global $bp;

	if ( bp_is_member() ) {
		$title = str_replace( $bp->displayed_user->fullname, $bp->displayed_user->userdata->user_login, $title );
	}

	return $title;
}
add_filter( 'bp_page_title', 'ray_bp_page_title' );