Go to account settings:

On the next page click on the change link next to Account Security:

That will show the "Secure Browsing (https)" option:

Make sure this is checked. If you do not have this option just check back later.

Enabling https will not noticeably impact the speed of either your computer or Facebook.

Why is this Important?

Because most users don't take appropriate precautions when using an open Wi-Fi hotspot they're frequently logging into those services and exposing their login credentials to anyone who might be in range. They can actually take over your Facebook account and use your private information in other ways

Used properly, https encrypts the entire conversation with the internet service - including your username and password. Anyone eavesdropping at your open WiFi hotspot will see only gibberish.

As it turns out, your login credentials for services like Facebook and others are often just as important to keep secure.

And now you can.

Mike