Check out Buffer's Event Marketing 101: How to Keep Attendees Engaged Before, During and After Your Event. Buffer demonstrates that social media activity before the event is a must-do. Some ways to engage in advance are promoting attendance, event reminders, schedule announcements, buzz from ticket buyers, and encouraging those who cannot attend to push those who are to share pics and info on platforms such as Twitter and Instagram.
There is a lot to learn from Buffer. For example, I learned to share behind-the-scenes pics well before the event begins, treat followers to a sneak peek back stage during the event, and use questions and polls to engage and interact with the audience during and after.