Bridge simply means any technology that allows them to connect things together. It could be cookies placed by one pixel images. It could be Adbe Flash settings. It could be the signature of extensions installed in your browser. You might think you only have common ones that millions of others use, but where did you get them from? The same "STORE" as everyone else. So you could have gotten an extension that contains some kind of serial number in it.
The EFF used to track fuckery like this but I think they have been comped by the clowns.
But the biggest thing is that FB and Twitter and Google are LARGE MONOPOLY BUSINESSES that function like a Communist government. They built tracking systems to track their own users and then realized that with a bit more flexibility they could use the same systems to track everybody even if YOU do not subscribe to their services.
They just bridged their systems and databases together.