Keep in mind that if you're used to viewing Facebook through the secure connection (https), you'll need to revert to the regular connection (http) in order to add Twitter to your Facebook page.
If you want to know whether your browser is on a secure website, the beginning of the web address will be https instead of http.
You can tell if they have a secure server if HTTPS appears in the checkout URL.
You can tell if a website's server is secure if it has HTTPS in the URL.
Look for a URL starting with https or a locked padlock icon on your browser before submitting financial details.