Re: It IS public data....
1) IS it public? Facebook lets you limit the visibility of your posts to your friends, or your friends' friends, or make them open to the world. If you have it set to something other than "the world" but this company still had access to info about it, then Facebook's privacy protections were a sham.
2) Even if it is public, going to millions of Facebook pages to extract this info would be very difficult and time consuming even after it is automated - and you'd need to keep updating the automation as Facebook tweaks its HTML. There's a huge difference between me writing down private information about myself and shoving it in a stack of papers 50' high that law enforcement has access to but would take them a lot of resources to search, and me typing that private information and having it put into a nice searchable database for law enforcement to conduct keyword or person searches in a millisecond.
Basically your argument is "since you are putting your information out there, it should be available for law enforcement (or anyone else) to do whatever they want with it". So I guess Facebook and Twitter shouldn't be used by activists to coordinate their actions? Even if you are against BLM, what about dissidents in countries like Iran or Russia who might "disappear" if they say the wrong thing about the wrong person? Social media has really helped them find others of like mind, and made the 'Arab Spring' uprisings possible in a way that they never could have been if they had to rely on people telling each other in person. What's their alternative, put up fliers on telephone poles?
While its nice Facebook/Twitter cut off their relationship with this one company - after it was outed - I wonder how many other companies they have similar arrangements with, and how many of them supply their info (willingly, knowingly, or otherwise) to law enforcement or the CIA?