What does 'twitter/facebook Town Hall' mean in Chinese?
A town hall meeting is known as 市民大会 or 市政厅会议. It has to be noted that this concept originates from America and may not be well received in other countries depending on the political system.

A Facebook or a Twitter town hall meeting can be generally termed as 网上市民大会. An example sourced from this news article reads:


  中新社旧金山4月20日电(记者
  刘丹)20日下午，美国总统奥巴马在加州帕洛阿图市的脸书(Facebook)总部举行“分担责任共享繁荣”网上市民大会时表示，希望更多有智慧有雄心的人留在美国，不希望看到下一个英特尔这样的企业在中国或法国创业。