Post Expo this year will take place on 18/19/20 September 2012, in Brussels, Belgium. The organisers have announced that the Hashtag will be #PostExpo and that this will be used on all Social Media.
There is already a Linkedin Group and a Twitter identity. Hopefully the organisers will also put up a Google+ page as well.
The twitter discussion at last years event was really good and I would strongly recommend that you go to Brussels with a Smartphone loaded with a Twitter App.