The USN stands for Unique Serial Number.
You already know it, spying you it's the easiest thing that facebook, email providers and any other website where you write information can do. In fact they don't "spy" it's you, you provide them with informations. On the other hand, you need to memorize every website with a different login / password combination, or, you compromise your security by using the same combo on every website. That's why we use the USN, so, without needing to log-you-in, you may attend any of the available events, and there is no annoying anti-spamming filter to pass by. This is innovation at its best.