Your onclick fires before the href so it will change before the page is opened, you need to make the function handle the window opening like so:

function changeLink() {
    var link = document.getElementById("mylink");

    window.open(
      link.href,
      '_blank'
    );

    link.innerHTML = "facebook";,
    link.setAttribute('href', "http://facebook.com");

    return false;
}