Under the 4th Amendment there’s what known as the “third-party doctrine.” This basically means that if you disclose information to a third party, you no longer have an expectation of privacy in that information and the 4th Amendment no longer applies. The classic example of this is jail-house informants or undercover snitches: if someone discloses information to another, that third party can turn around and disclose to the government. This doctrine has essentially been extended to include “wrapper” or “envelope” data in conventional communications: you disclose the name and address on the outside of envelopes in order to get letters delivered, so this address information is not protected by the 4th Amendment even if the contents of a letter are; similarly, when you dial a telephone number you disclose the telephone number to the phone company, which needs this information to collect the call, so phone numbers are not protected. This is essentially why metadata of electronic communications within the US is being collected: it’s the digital equivalent of wrapper communications.
At any rate, the 4th Amendment wouldn’t even apply to these cases (nor are FISC warrants needed) because this is not surveillance targeting individuals within the US.
