It doesn't matter if people are happily "handing over their data" - the government still needs a warrant to request the information. In United States v. Warshak it was ruled that a person has a reasonable expectation of privacy in his/her emails and that the government violated Warshak's Fourth Amendment rights by compelling his internet service provider to turn over his emails without first obtaining a warrant.
That's very true, but as long as the company is doing the 'surveilllance' and not the government, the warrant can be done on a just-in-time basis. This essentially all that Prism is, after all.
