Connecting your account to Facebook
From Legacy Roblox Wiki
You can connect your Roblox account to Facebook. When you do this you gain access to a few cool features.
How do I connect my account to Facebook?
First of all, you'll need to get a Roblox account if you don't already have one. Then, go to your homepage and scroll down until you see
Click the blue "Connect" button to go to a page that asks you if you'd like to allow Roblox the ability to post things on your wall. You may have to log in to facebook if you aren't already. Then, when you scroll down on your homepage, you should see this: