[lacker]

If they break the script, they'll probably break the unfollow UI for legitimate human users

Messing with this is a lose-lose for Facebook.

Nah, this sort of thing really happens all the time. Think hundreds of scripts like this automatically disabled each day. This one just flukily got some press attention. Large tech companies will have teams entirely dedicated to preventing scripts from doing things while keeping the site running as normal for regular users.