It is not essential that they sort my feed, though. Everything still works perfectly fine with a simple chronological feed.
None of the tracking they do is essential to the service they ostensibly provide to their users, namely as a microblogging/discussion/sharing platform.
Why do you think ranking is important? A simple chronological news feed should be perfectly fine.
Sure, rank search results based on how many users a given group has, and put the most popular ones at the top. That doesn't require violating anyone's privacy.
And they are certainly allowed to do ranking. But they can only do it on non-PII data, on data that has been deliberately made public, and on data which people have consented to its collection and storage with unequivocal opt-in.
None of the tracking they do is essential to the service they ostensibly provide to their users, namely as a microblogging/discussion/sharing platform.