If you wish to link your account with your Facebook account, this must be done when you create your account. When you go to sign up on the homepage, select the Continue with Facebook button:
To log in with your Facebook account, you must authorize access to your Facebook profile by entering your Facebook credentials.
By having your account linked to Facebook, this allows you to use your Facebook email and password to sign into the social network either on a computer or mobile app.
Your posts will not also be visible on your Facebook page.