You make your own rules about what matters.
If you spend 15 minutes only eating food, chances are you'll pay more attention to what you eat. You may be eating slower than you normally would. You may be chewing more. You may even notice how much you like (or don't like) what you're eating. And that matters.
For the 15 minutes of time, doing something that requires acting in the physical world somehow can be pretty satisfying. Social media friends might be important to you, but so might actual humans, and Facebook never seems to have any problem finding it's way into the day. If you can, find someone real that you like to Like during lunch.