The phone company is the best example because with a couple of alligator clips they could easy listen in on phone conversations. It's understood that listening in is a rare occurrence but we are ok with it happening.

Same goes for FB, Google and other providers. They have the ability to read your mail and occasionally do just like the phone company.

Bonus example: voice mail. It's just like email since its a message sitting on the service providers computer. Can the Feds just listen to those messages?