Mark Zuckerberg is now taking questions both physically and on the 'Q&A with Mark' Facebook page and answering them live. Questions can be submitted via the comments on the Facebook page and voted on via likes on the comment.
So far questions have included things such as whether or not Facebook plans on dominating VOIP and Messaging in the mobile space, as well as questions about whether or not "Facebook is losing its charm and becoming boring."
You can watch the stream here and post your questions directly in the comments. Unfortunately the stream cannot be embedded so you must watch it directly within the Facebook post. If you miss the stream, it will be posted afterwards for those interested.
Update: The Q&A has finished, but you can follow the same link and stream the session.