Wonder why people like using Facebook? There are a lot of other ways to communicate online, such as email, instant messaging, etc. What makes Facebook unique is the ability to connect and share with the people you care about at the same time.
For many, having a Facebook account is now an expected part of being online, much like having your own email address. And since Facebook is so popular, other websites have worked to integrate Facebook. This means you can use a single Facebook account to sign in to different services across the Web.