I think traffic is down becuase the number of topics are down. Facebook/twitter/etc. do not do what a community forum can do. You and your moderators need to seed topics, posting about events, news, issues, and then let the community respond. Use the other social media tools to drive people back to the forum (which you control and not have to depend on Mark Z)
It is a chicken/egg thing. People stopped coming, as the level of topics went down. Once the traffic went down, so did the topics.
Look back at the recent multi-pager topics and you can see a good topic drives people to the forum.