Whether its against the spirit of the constitution depends on how you perceive the nature of monitoring. The "spirit" of the 4th amendment is to keep the police from invading your personal space. If you think of your gdrive as a natural extension of your desk drawer, then I can see how you could interpret it as against the spirit, because the NSA looking through those documents is like the police rummaging through your desk.

That's not the only way to view the situation, though. That's the "software" view of the world. I take a "hardware" view of the world. That data is sitting on Google's property. Its servers, its hard drives. You've given up posession of that data. Under that view, rummaging through that data is neither unconstitutional nor against the spirit thereof.

more generally, the constitution just has nothing to say about the larger issue of "tracking." The founders wrote the 4th with a specific grievance in mind: British searches of houses with very broad warrants. They just didn't address the government collecting information you left out in the world to track you.

You have rented, by agreement with Google, a portion of the space on their servers. It's no different than a physical storage locker at a storage facility. Yes, that is property that is owned by a third party, but you've still entered into an agreement for the exclusive use of it and you have an expectation of privacy. Police are not allowed to search it without a warrant.

If a bank loses your money, you can sue them. Can you sue google if they lose your data? No. So, no, its not the same. They have very few legal obligations with regard to your data (otherwise, instagrams recent copyright policy change could be considered the tort of conversion: the illegal use of someone else's property for personal profit). One can imagine a legal regime wherein a gdrive is "your property" like a safe deposit box, but that's not what exists right now.

As for how Google isn't a modern mailman: its not an organ of the government (4th amendment protections apply only to postal mail, not say UPS); and it reads your mail unlike the postman.

Its pointless to speculate what the founders would have done had they addressed the issue. They didn't, and there are crucial distinctions that make the protections they did address inapplicable to online data. Given that, the solution is either a privacy amendment to the constitution or sweeping legislation.