$watch not being triggered on array change

I'm trying to figure out why my $watch isn't being triggered. This is a snippet from the relevant controller:

$scope.$watch('tasks', function (newValue, oldValue) {
    //do some stuff
    //only enters here once
    //newValue and oldValue are equal at that point
});

$scope.tasks = tasksService.tasks();

$scope.addTask = function (taskCreationString) {
    tasksService.addTask(taskCreationString);//modifies tasks array
};

On my view, tasks is clearly being updated correctly as I have its length bound like so:

<span>There are {{tasks.length}} total tasks</span>

What am I missing?