Only the Messenger applications for iPhone, iPad, and Android can have secret discussions.  The secret conversation functionality isn't available on the website or desktop version of FB Messenger. If you're using an OS compatible with this add-on, you can engage it by adhering to these steps: Start the Messenger application and the compose message button on the top-right side. You'll see a rundown of all your contacts. Here, press the top-right tab; on iOS, it will read Secret; on Android, it'll appear as a padlock-themed icon. Select the people with whom you wish to hold a private discussion. You can create hidden group chats; it must be a private conversation. You and the people you choose will have a private discussion. Your message will show up on their smartphone immediately after you send it.