I love Larry Page’s “toothbrush test”. The toothbrush test goes like this: a service passes the test if it is useful enough and important enough that people will use it at least once or twice per day. I believe Page was using this as a test for companies Google should be looking to acquire. This test can be applied to all sorts of services: social, utility, search, mail, productivity, messaging, communications, networking, security, home, wearables, internet of things, &c. What an incredibly clever way to summarize how valuable a service truly is.
A smaller variation of such a test is perhaps one I woke up with last week: your first service of the day. This won’t work for all types of services, and likely is only good for messaging, social networks and some types of personal productivity apps.