Some websites embed Facebook plugins, e.g., comment with Facebook, Facebook messenger live chat, etc.
The first time I visit one of those sites, they already know my currently logged in Facebook account.
How is that possible technically?
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Because Facebook knows your browser. When you logged in, it almost certainly set cookies, but it can identify your browser in other ways, like your IP address, installed add-ins, et cetera and so on.
Since the plugins are also served from Facebook's domain(s), it can "see" your browser when you visit other sites, so it knows it's you.