Facebook is trying to kill the Chrome extension Unfollow Everything
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>Here's how Facebook tries to kill it
This thread doesn't answer "how Facebook tries to kill it" but might answer "why"
Without seeing the cease and desist it's really not possible to assess this. It could be that Facebook is overreaching or it could be that he copy and pasted proprietary code. We don't know.
IMO this is no different than asking InfoSec researchers/pen testers to not touch their endpoints.