If I was Mark my answer would be “or what?” These people already work at a vile company. Which means they sold out already. If what mark does to other people doesn’t bother them they probably won’t have the backbone to leave if he says pound sand. “Oh it’s ok if you do it to other people just not me.” Get bent.
If this is what you get for selling out, just imagine what those of us who didn't sell out can expect? People sell out in order to get better conditions for themselves. Furthermore, this monitoring only strengthens Meta's ability to effectively surveil the rest of us.
His answer was “or what?”. And then enough people they cared about quit, and now you’re seeing the articles about them apologizing, saying they care, etc.
So the petition didn’t work, exactly, but served as a warning: employees said, fix this (or else). Mark said “or else what?”, and enough people said “I quit” to make them look bad.