There are many ways to hack facebook accounts.One them i will describe you briefly below.
1: Facebook Phishing:
In this method hacker actually uses the page look same like the facebook and ask you to login, but the domain of this page is registered on the name of hacker. So, when we try to login using this page our Facebook Credentials, the hacker will get the these credentials we entered in the "Username" and "Password" field. Hackers actually make you fool by asking you to approve your account or to verify it.
Prevention:
The only thing we have to do is that never try to login with your Facebook account on a untrusted website. Hint: The address of phishing page is not same as the www.facebook.com. It may be different. And never trust your friends.
