What I would do is go to https://developers.facebook.com/docs/plugins/ first. There you can find all the “social plugins” that are offered by Facebook. You can choose the one you want and customize it to your liking. Then they will give you the “embed code” for your site.
Copy that code and then go create a text widget. Paste the embed code into the text widget and save your changes. That should add the Facebook item directly to your site.