In fact, the reason I hate Facebook and social media so much is that they enable patently stupid theories like this to gain traction.
And I don't see anything illegal here. Facebook.com is a private website of Mark and stakeholders, they can do whatever they want on their territory (if it's legal). As much as you can do on your backyard.
Ethics is a different topic, but it's perfectly legal.
Like if someone said "facebook tracks you everywhere you go" or "your phone is listening to you" or "your television watches everything you watch" it would be shrugged off.
Looking forward to the day that Google, FB and Twitter are classified as public-utilities