(Tedious disclaimer: my opinion, not my employers. Not representing anybody other than myself. I work at Google, not on cloud, although I routinely fire my nerf gun at people who do)
An example for you: Microsoft used to have an identity service called 'Passport'. They wanted to make it into a universal login, along the lines of Google ID or Facebook logins today. They created a whole suite of related services, codenamed 'Hailstorm', which they tried to bring to market under the terrible name of ".NET MyServices". And they failed because the universal reaction of the world was that they were not prepared to trust Microsoft with their privacy. (see, e.g. http://www.theregister.co.uk/2002/12/17/net_my_services_gone...)
If any of these are not met then you are trusting someone else with your privacy.
https://upload.wikimedia.org/wikipedia/commons/c/c7/Prism_sl...