Imagine if the bank had the lock to your lock box, and bank employees had access to the contents of your lock box, and bank employees regularly rifled through the contents of your lock box. Also, the bank had no obligations as to your lock box. You couldn't sue the bank if they lost the box, or if someone stole the contents, etc. To me, all that would make the box seem a lot less "private."
The 4th amendment doesn't extend to say the stuff you store in your friend's garage. Google, Facebook, etc, accounts resemble that a lot more than they resemble bank safe deposit boxes.
Also, it is not like a friend's garage; they aren't hosting people's email as a favor. It is more like a rent-a-storage-room place with a peculiar method of paying the rent; there is a business relationship/transaction going on. We trade algorithmic-advertising access to our emails in exchange for them providing hosting.