It's been tricky for me to figure out how to make singleton behavior in polymer (1.x) .. I'm probably getting old ;) .. anyway - here goes: A tick (1 sec interval) tock (1.5 sec interval) counter service.

app-data.html

<link rel="import" href="../bower_components/polymer/polymer.html">

<dom-module id="app-data">
	<script>
		(function() {
			console.log("This should only run once...");
			var listeners = {
				tickListeners: [],
				tockListeners: []
			}
			var refcount = 0;
			var g_data = {tick:0, tock:0};

			setInterval(function(){
				g_data.tick++;
				_tick_changed();
			}, 1000);

			setInterval(function(){
				g_data.tock++;
				_tock_changed();
			}, 1500);

			var _tick_changed = function() {
				//console.log("data tick changed", g_data.tick);
				listeners.tickListeners.forEach( function(instance) {
					instance.notifyPath('data.tick');
				});
			}

			var _tock_changed = function() {
				//console.log("data tock changed", g_data.tock);
				listeners.tockListeners.forEach( function(instance) {
					instance.notifyPath('data.tock');
				});
			}

			// event handlers
			document.addEventListener('event-app-data', function(evt) {
				//console.log('evt received: ', evt);
				if (evt && evt.detail && evt.detail.message) {
					switch(evt.detail.message) {
						case 'resettick':
							g_data.tick = 0;
							_tick_changed();
							break;
						case 'resettock':
							g_data.tock = 0;
							_tock_changed();
							break;
					}
				}
			});

			Polymer({
				is: 'app-data',

				properties: {
					data: {
						type: Object,
						value: g_data,
						readonly: false
					},
					key: String
				},

				created: function() {
					console.log("[app-data] created", this);
					refcount++;
					console.log("REFCOUNT:", refcount);
					key = this.getAttribute('key');
					if (!key) {
						console.log(this);
						throw('app-data element requires key');
					}
					switch(key) {
						case 'tick':
							listeners.tickListeners.push(this);
							this.async(function(){this.notifyPath('data.tick')});
							break;
						case 'tock':
							listeners.tockListeners.push(this);
							this.async(function(){this.notifyPath('data.tock')});
							break;
					}
				},

				detached: function() {
					console.log("[app-data] detached", this);
					key = this.getAttribute('key');
					var arr
					switch(key) {
						case 'tick':
							arr = listeners.tickListeners;
							break;
						case 'tock':
							arr = listeners.tockListeners;
							break;
					}

					if(arr) {
						var i = arr.indexOf(this);
						if (i >= 0) {
							instances.splice(i, 1);
						}
					}
				}
			});
		}());
	</script>
</dom-module>

Note: I know it's probably very hacky and most probably very non-polymer ... but seems to work and I'd like some feedback on how I can make it better. ALL other examples of "singletons" I have seen regarding polymer has relied on some form of data replication. This may work fine for small state observers - but not when you need e.g. a USB serial hardware manager that should be connected to as a service from potentially many different web component elements in different parts (visible and otherwise) of a rich single page app. Some controlling parts of the hardware, some listening on low frequency data and some relying on fast streaming sensor data coming through the same service component - that ALSO must be able to provide polymer data binding. Anyway, as I mentioned in the description: I might just be getting old :)