Why wouldn't they? The publicly exposed data is publicly exposed. Is there any reasonable sense to privacy at that point? The more interesting question is how much data do they get that is "private" without a warrant.
The following are a few examples of how the private data may be being gathered by the FBI, under existing legal structures.
There is also the possibility of voluntary cooperation by corporations, or even corporations selling commercial access to their data (Facebook, Cellular Carriers).
The following are a few examples of how the private data may be being gathered by the FBI, under existing legal structures.
https://www.nytimes.com/2017/01/12/us/politics/nsa-gets-more...
https://en.wikipedia.org/wiki/Room_641A
https://en.wikipedia.org/wiki/National_security_letter
There is also the possibility of voluntary cooperation by corporations, or even corporations selling commercial access to their data (Facebook, Cellular Carriers).