Background information on the NSA’s PRISM program:

• Electronic Frontier Foundation:  What We Need to Know About PRISM
• The Volokh Conspiracy:  NSA and FBI Have Real-Time Access to Major U.S. Internet Companies to Track Individuals Outside U.S.
• Electronic Frontier Foundation:  The NSA’s Word Games Explained: How the Government Deceived Congress in the Debate over Surveillance Powers

Third-Party Doctrine, from what I gather the primary argument for PRISM not being a grand-scale 4th Amendment violation:

• Techdirt:  Why The ‘Third-Party Doctrine’ Undermines On-Line Privacy Protections
• ABA Journal:  The Data Question: Should the Third-Party Records Doctrine Be Revisited?
• Bloomberg.com:  How Rand Paul Can Take On the NSA