<?php
function alter_table(){
	global $wpdb;
	$sql = "ALTER TABLE  ".$wpbd -> posts." ADD  `likes_count` INT( 11 ) NOT NULL DEFAULT  '0'"
	$wpdb -> query($sql);
}
function on_create_blog( $blog_id, $user_id, $domain, $path, $site_id, $meta ) {
	switch_to_blog( $blog_id );
	alter_table();
	restore_current_blog();
}
add_action( 'wpmu_new_blog', 'on_create_blog', 10, 6 );