Let's take a simpler situation. Say you tell me that you were with Bob and Jim out on Mill Road last week. Now if a crime has been committed, and a cop is asking questions door to door if anyone knew where you were and who you were with, do you think that the officer should have a signed warrant for me to disclose to them that you said you were out by Mill Road with Bob and Jim? How would the Facebook situation be any different, other than the fact that they make money off you telling them these things?

> Now if a crime has been committed, and a cop is asking questions door to door if anyone knew where you were and who you were with, do you think that the officer should have a signed warrant for me to disclose to them that you said you were out by Mill Road with Bob and Jim?

It's important to distinguish between you being allowed to tell them and them being able to compel you to tell them. I see no problem with requiring a warrant for the second one.

A warrant isn't required to compel someone to testify against someone else. Historically,[1] the balance has been struck in favor of the government's right to all relevant evidence.

[1] The subpoena ad testificandum dates back about 800 years.

I get that judges are resistant to changing something that has existed for a long time, but it still seems like a bad policy. If someone has to be compelled to testify it will almost always be either because the witness doesn't want the perpetrator to be punished or the perpetrator is so dangerous that the witness is afraid to testify against them. Compelling testimony in either circumstance (as opposed to, say, negotiating for it in exchange for witness protection) is very likely to produce false testimony. It's hard to see a government interest in false testimony. Moreover, history is not always right. Censorship is as old a practice as the printed word, that's no excuse not to apply the First Amendment to curtail it.

This is also justifying the lack of a warrant requirement based on something that breaks the analogy to the original case. Witness testimony is no one's "papers and effects", unlike stored data.

Doesn't withholding potentially pertinent information from a criminal investigation constitute obstruction of justice?

How do you expect the courts to respond if the government prosecuted AT&T for obstruction of justice for refusing to execute a wiretap without a warrant?

Strictly speaking they don't "make money off of you telling them these things." They make money from advertising in a well-targeted fashion, which is actually a much more complicated proposition, and one that has far more to do with how much sense they can make of the the data you provide – not just the acquisition of the raw material.

Using a more accurate view of their model, it becomes clear that FB is strongly incentivized to keep their raw data to themselves. Failure to do so means risking that someone will not only beat them at their own game, but will do so at their expense. Same goes for Google, etc. So yes, it is perfectly reasonable to share a lot of very personal data with them while expecting that much of it will be carefully protected. After all, the stratospherically high value of their position depends on the inability of others to connect the very fine-grained demographic profiles they create with the specific individuals behind them.

Given this reasonable expectation of privacy, it makes more sense to see these third-parties more like doctors, priests, or accountants. Yes, they're all third parties too. But they're ones who we absolutely expect to protect our privacy. Given how deeply established the norms of confidentiality are in cases like these, I fail to understand why we we've acquiesced so readily to the fairly radical standard set in 1979.