They don't need to store the traffic. All they need is to store the endpoints and timestamps and you could compress that quite well to have usable information, such as who talked to who and when.
Your actual data is just a subpoena away anyway in most cases.
Money are of no objection to that industry. The combined budget of NSA, CIA, FBI, Pentagon, foreign bases, Afghanistan and Iraq occupation, etc. is in the trillions. Why trust FB or Google if all it costs to capture and store the information are money.
NSA is an intelligence agency, not a law enforcement agency. They don't operate within the court process. Which means they don't really do subpoenas or warrants. Furthermore, any data covertly collected by the NSA would be inadmissible in court anyway, which would make it impossible to get a subpoena or warrant based on it anyway.
Your actual data is just a subpoena away anyway in most cases.