In a huge keynote, audience questions are almost impossible. The practicalities of picking who can ask a question, getting a microphone to the person. The lottery of whether the question will actually be interesting or relevant, and/or whether the person is actually good at asking to the point questions. And what if no one actually responds to the famous speaker inviting questions?
Dedicated smartphone apps (or even Twitter) seem to solve part of these problems. Users have to be brief, don’t interrupt the live presentation, people can upvote things, and you can pre-populate question to get people started.
I have seen them in action. Often the questions are projected on a huge screen behind the speaker. But, that constantly changing huge screen is actually distracting, there is even a possibility of “background vandalism” for controversial speakers, and most of the times, the questions are actually ignored.
A solution? Use the system, but don’t put the questions on the main screen. Answer at least one question. But most importantly, use the questions that pop up to make other conference presentations more relevant. Questions are live feedback about what the audience actually wants to hear. If it cannot be used now, maybe it can help the next session.