Check out Pardot if you want to get an idea of what's possible. They drop a cookie on you the moment you browse the site and it logs every interaction you have with the site. When you finally sign up or fill in a form somewhere it'll associate those sessions with your new account and let you better target drip campaigns and market to them.
There's a lot of other ways to identify someone. It's like in Serenity -- everything has a fingerprint.