If Someone knows your password or someone manages to break into your account you'll be known by checking your active sessions.
For it Goto Account Settings--Security tab--Active Sessions.
But it's better to enable "Secure Browsing(browse through HTTPS Connection) and Email and Text Notifications".
By enabling email and text notifications each time you logged into your account you'll get a mail to your email-id and text message to your mobile.
For it Goto Account Settings--Security tab.
Before Enabling the notification settings you must change your password.Now comes the headache that the person who managed to break into your account does't change the password, then you can directly go to Account Settings--General--Password then change it. Otherwise Click on forget your password
It will take you to identify your account
Now enter your Details and get it back.
The main things is that to enable secure browsing and enabling the notifications and also check the open sessions.
