Ivan wants to know what he's giving away when he logs into a site using his Facebook ID. Leo says that's called Single Sign-on, which makes it easier to sign in. Many services, including Google and Twitter also offer it as a convenience. It's a user verification system that doesn't require him to create an account, nor does it give them access to his account. But it gives Facebook, Google, and Twitter access to more information about where he visits. It's safe to use it, but if he's concerned, he can create a dummy account that he'll only use for that purpose.