You hit a really important point here: 3rd parties have held personal information since the dawn of civilization: messengers, small-town post offices, the US Mail, Ma Bell, stores that tracked purchases against your line of credit in a ledger, etc.
A computer using keywords in my emails to display relevant advertising is orders of magnitude less scary than a town gossip who runs the post office and tells my extended family and neighbors anytime I receive a letter that might indicate I'm deviating from social norms or religious morals.
The code running Amazon doesn't judge people for buying erotica; the town bookseller does. The card catalog won't out you to your friends for looking up LGBT reading material; the librarian might. The code running Google Maps doesn't care and won't tell anyone that you're driving to a woman's house and then a restaurant or bar most Saturdays; the waitress/bartender will almost certainly tell a friend/stalker/jilted lover/cop who you were with.
Big Data is far less threatening than Little Data.
I don't get to commit murder even if I feel it's justified; Amazon doesn't get to refuse to comply with a court order even if it feels it's unjustified. Nobody, individual or corporation, is going to sacrifice their well being or sit in jail indefinitely for contempt of court to protect the privacy of your purchase history.
The only people who really pull that stunt are journalists.
The threshold for obtaining a subpoena is remarkably lower than that. NSLs -- I wish I could tell you, but the whole program is classified.
The EFF reports that mobile providers received 8 million law enforcement requests for GPS data: https://www.eff.org/deeplinks/2009/12/surveillance-shocker-s...
And ... due process like subpoenas and court orders? Not so much:
AT&T said it now responds to more than 700 request a day, about a third of which do not require court orders or subpoenas.
http://news.cnet.com/8301-1035_3-57468316-94/cell-carriers-s...
I'm all for rule of law. This isn't.
Government is somewhat closed, but it also has an army and isn't going anywhere soon. Facebook is not trustworthy as a single source of truth.
Switching banks is inconvenient too, especially if you have outstanding loans.
If you have a Facebook group where personally identifying participants isn't desirable (i.e. you would want non-FB users participating) than you're probably looking for a wiki or Google Sites instead. Facebook has every right not to be a web publishing tool.
And my point still stands that the overwhelming majority of people are comfortable with their most private data being in the hands of third parties. And Facebook has long reached that point where people can trust it for managing their semi-private data.
Because at the end of the day user experience is order of magnitudes more important than some arbitrary geek obsession with being open. Which isn't important for most people since they are used to having their personal information owned by third parties e.g. banks, employers, insurance companies, government.