Purpose. What is the purpose of the website? It should be clear that your website should have a purpose; that purpose should be something different than what you would already expect from Facebook could deliver. People, nowadays, almost always just look in Facebook in case they wanted to shop for stuff, or just look around. Give people something they can't do on Facebook yet they can do on your website. On top of my mind: when shopping for things online, they can log in to your portal, add items to their baskets, remember the details of the account owner like the delivery address, payment details, coupons, etc. Be able to watch out for what's hot and trending, what's on sale, which items were also viewed and bought, etc. These are things you have yet to see on Facebook's features. People are given the website's purpose so instead of coming to Facebook for that, they just go to your website since it's already established that they can do it on your website with much more ease.