How to use the wpmublogsaction hook?

Researching for this action hook, I found this forum thread and as it is closed, I’m opening a new one to report back.

David Sader’s code has to be improved. The blog id is passed as the callback parameter and instead of ABSPATH, use the more reliable WP_CONTENT_DIR:

add_action( 'wpmublogsaction', 'b5f_upload_space_used' );
function ds_upload_space_used( $blog_id )
{
    $dir_name = WP_CONTENT_DIR . '/blogs.dir/' . $b_id . '/';
    $size = round( get_dirsize( $dir_name ) / 1024 / 1024, 2 );
    echo '<strong>'. $size . '</strong> MB';
}

To give the column a custom name, we use the following:

add_filter( 'wpmu_blogs_columns', 'b5f_rename_actions_column' );
function b5f_rename_actions_column( $sites_columns )
{
    if( isset( $sites_columns['plugins'] ) )
        $sites_columns['plugins'] = __( 'Space used' );
    return $sites_columns;
}