I'll wade in without any facts and figures here, but you probably won't find it too difficult to hunt down the stories that support the view.
Unfortunately thanks to the cynical exploitation of open borders, financial privacy, the amateurism of our law enforcers and the laxity of punishment for drug offences here, Belgium is pretty much an open house for drug trafficking.
It should be noted however that, although Belgium may be both a destination and a crossroads, very few of the dealers and users are actually Belgians, which means that somebody else is doing all the moving and shaking around here.