Here's a directive that will add target="_blank" to all <a> tags with an href attribute. That means they will all open in a new window. Remember that directives are used in Angular for any dom manipulation/behavior. Live demo (click).
app.directive('href', function() {
  return {
    compile: function(element) {
      element.attr('target', '_blank');
    }
  };
});
Here's the same concept made less invasive (so it won't affect all links) and more adaptable. You can use it on a parent element to have it affect all children links. Live demo (click).
app.directive('targetBlank', function() {
  return {
    compile: function(element) {
      var elems = (element.prop("tagName") === 'A') ? element : element.find('a');
      elems.attr("target", "_blank");
    }
  };
});
Old Answer
It seems like you would just use "target="_blank" on your <a> tag. Here are two ways to go:
<a href="//facebook.com" target="_blank">Facebook</a>
<button ng-click="foo()">Facebook</button>
JavaScript:
var app = angular.module('myApp', []);
app.controller('myCtrl', function($scope, $window) {
  $scope.foo = function() {
    $window.open('//facebook.com');
  };
});
Live demo here (click).
Here are the docs for $window: http://docs.angularjs.org/api/ng.$window
You could just use window, but it is better to use dependency injection, passing in angular's $window for testing purposes.