Because Facebook lets the FBI access everyones information without a warrant. Why can they do that? Because back in the 1970's the police figured out who some crazy stalker in Texas was by asking the phone company whose phone number was calling some rando lady and the court said that info was not his, it was the phone companies and a warrant was not needed.
So because of one autistic sped in Texas 40 years ago the government claims they no longer need things like warrants or probable cause to seize every iota of information related to your digital activities.
Depending on how contrarian
@Null is feeling, he could in theory challenge the whole supposition and do what that Phone Company should have done all those decades ago and demand an actual warrant rather then a strongly worded demand for evidence production. Companies like facebook never oppose subpoenas because why would they? The Government would just get pissy with them and cost them more money. Of course, a big multinational like Facebook is in a much better position to have a dick measuring contest with Federal Prosecutors then the KiwiFarms is, and the heroic stand would most likely just end in a full butt fucking.