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He dug deeper, past the layout logic, looking for the communication layer. He knew Facebook used a system called MQTT or WebSocket for real-time chat. He wanted to see if he could see the "Seen" status trigger before the UI updated.

Elias opened a new tab and went to his News Feed. He hovered over an ad for a pair of hiking boots he had looked at three days ago on a different site. He didn't click. He just hovered for two seconds. view page source facebook

He found it.