If you are using Safari and want to know how to deactivate Facebook on iPhone device then open Safari and click on three horizontal lines. Scroll down and click to settings. choose account ownership and control from your Facebook information section. Choose deactivation and deletion. Click to deactivate account and continue to account deactivation. Lastly, choose from the available list of reasons and click to continue. Now, confirm and your account will be deactivated until you choose to log back by using the app or in a web browser.