Lazy singleton with BackboneJS
This is not the simplest way. 


There seems to be no point in instance, if there can be only 1 instance, why have it all ?
Why have initialize at all ?
Why set foo to 0, I would set it undefined
If I were to keep the instance concept, I would write this:

_instance: null,
get instance() {
    if (this._instance === null) {
        this._instance = new _LazySingleton();
    }

    return this._instance;
}


as 

_instance: null,
get instance() {
    return this._instance = this._instance || new _LazySingleton();
}



I would probably go for something like this ( I might have the syntax wrong )

var LazySingleton = Backbone.Model.extend({
    defaults: {
        foo: undefined
    },
    initialize: function(){
    });