You can run kiosk mode and there are a few other options. Each of these makes it more likely that they'll be on Facebook on their phone during that time, rather than looking at both Facebook and the course on the computer screen. There is no technical measure to ensure that they are looking at the material. Come to think of it, people get up to get a sandwich when the Dallas Cowboys cheerleaders come on the screen, so there's probably no non-technical way to force someone's attention either.
Do you want the students to gaze at the material, learn the material, or know the material? If so, you might consider checking to see that they know it (get a 90% on the pretest and you're done) or that they learned it.