A lot of good points about why it's useful to have Twittering- people can "be there" virtually, instant commentary, and questions regarding the presentation content can be answered by a mulitude of systems.
From there the article started to go down the path of adopting more systems to enhance communications during a presentation (referred to as backchannel), and so did the comments.
When I was in school it was called raising your hand. Whiskey. Tango. Is that a completely foreign thought here?
If you want more participation from the audience, engage them and allow questions. Make being there worth it. If it was really important for someone to be there, they would be.
Remember: You determine the level of your involvement.