r-a-y

function ray_bp_get_activity_action( $action, $activity ) {

    $displayed_user = bp_core_get_core_userdata( $activity->user_id );

    return str_replace( '>' . $displayed_user->display_name . '<', '>' . $displayed_user->user_login . '<', $action );
}
add_filter( 'bp_get_activity_action', 'ray_bp_get_activity_action', 1, 2 );