<?php
// Creating tables for all blogs in a WordPress Multisite installation
function on_activate( $network_wide ) {
    global $wpdb;

    if ( is_multisite() && $network_wide ) {
        // Get all blogs in the network and activate plugin on each one
        $blog_ids = $wpdb->get_col( "SELECT blog_id FROM $wpdb->blogs" );
        foreach ( $blog_ids as $blog_id ) {
            switch_to_blog( $blog_id );
            create_table();
            restore_current_blog();
        }
    } else {
        create_table();
    }
}

register_activation_hook( __FILE__, 'on_activate' );
?>