This story should cause outrage among the users and developers (and any
potential users and developers) of any software that is like Social Fixer, that is, any software that stores, manipulates, and presents proprietary data in ways that the "owner" (e.g. Facebook) of said data did not intend, for the use of a first party.
The thing that makes the FB legal threats against Social Fixer different, and more troubling than when 3rd party services are shut down, is that Fixer is a "first party" piece of software. It's not helping users syndicate information, or reselling it, or letting anyone else even look at it or use it. I'm just organizing the data that I do have into a format that is more useful to me.
If you accept that there are limits on what we do with our data, where does it stop? Am I allowed to download pictures of my friends and view them in programs that do not display ads? Am I allowed to store and search my own messages, or my friends' messages, in a program that does not notify Facebook or display their ads? If I have an Excel Spreadsheet with friends addresses and phone numbers gleaned from Facebook profiles, have I broken the law?
I can understand Facebook's concern. If it suddenly became common to run programs like this over Facebook's site, then the "user experience" will suddenly get completely out of FB's control. People might not become immune to tracking, but neither would they no longer be manipulated by feed algorithms.
FB has about as much right to control what I do with the messages my friends send to me on Facebook as Google has to control what I do with my email. It doesn't matter if the "messaging protocol" is screen-scraped from the DOM.
The key over-reaching provision is this gem: "You will not do anything that could disable, overburden, or impair the proper working or appearance of Facebook, such as a denial of service attack or interference with page rendering or other Facebook functionality."[1]
Does changing my browser window size "interfere with page rendering"? What about changing font? Or user styles? Or Ad Blocker? What if I switch browsers? What if that browser renders Facebook poorly or not at all, am I in violation? If I edit the DOM in Firebug or Inspector should I get sued?
Going after people who make tools that help me organize, manage, and control my consumption of the Internet firehose is absolutely wrong. If the Justice system supports Facebook's claim (which very well could happen) then the only alternative is to stop using Facebook, and any other 3rd party that claims not only ownership of the data you disseminate on it, but claims the right to limit your ability to organize and read your own inbox.
[1] https://www.facebook.com/legal/terms Section 3.11