When I click on the Facebook bookmark I saved to my home screen, m.facebook.com appears in Safari but the navigation bar at the bottom and the URL bar at the disappear. At first I thought I was launching the Facebook app by mistake but I verified that's not the case. Any idea why this happens?
Yes, mobile sites can be programmed to remove the navigation bar whilst browsing the site. Makes for a better user experience when these elements get out of the way.
In particular the apple-mobile-web-app-capable element.