Analytics and ad operators, social media firms, and other data exchanges can identify you using this, and they can use this information for tracking what you say and do online.

Verizon has been in business with these companies to sell your identity to them -- Verizon is effectively including your name, address, Facebook account, and email address with every HTTP request you make. Again, this applies across all browsers and devices, and you cannot opt-out.

In some sense it's a bug in client software, that allows information to be stored for eventual inclusion in future requests (i.e., like a regular browser cookie), but outside the ability of the user to monitor or delete (unlike a regular cookie). For some time people mostly blamed this on Flash, but there are probably other ways for old and/or strangely-configured clients to screw up like this.