Given a large audience, you cant expect every single person to be listening - some maybe fidgeting on their mobile device (another term for when somebody's hand moves around suspiciously in their trouser pockets), busy digging nose or they could even be asleep ^^;
Its important to stay focused when you spot this and remember one thing - the people who are not paying attention dont matter - people who *are* listening *do* matter. Focus on the folks who are listening or what will end up happening is that your performance drops on stage and you could end up making *everybody* fall asleep ^^;
This rule can be applied in real life - people who dont get it dont matter - people who do get it do matter. Dont waste time trying to make people care - focus your time on people who already do care.