register_activation_hook( __FILE__ 'my_plugin_activation' );

function my_plugin_activation( $network_wide = false ) {

    global $wpdb;

    if ( $network_wide ) {

        $blogids = $wpdb->get_col($wpdb->prepare("
            SELECT blog_id
            FROM $wpdb->blogs
            WHERE site_id = %d
            AND deleted = 0
            AND spam = 0
        ", $wpdb->siteid));

        foreach ($blogids as $blog_id) {
            switch_to_blog($blog_id);
            my_plugin_install();
        }

        restore_current_blog();

    } else {

        my_plugin_install();

    }

}

function my_plugin_install() {
    /* Setup your tables here */
}