The only spamming is now being happened by the ANN thread bumper which can only be controlled by people who says many are still spamming. There is nothing more to do without reporting those in a constructive way.

Although we can't say it fell drastically, it was quite big amount. Anyway, it may be because of inactivity of merit sources in last week (not sure though) or may be because of merit deserving posts or removing one of the large merit source as you mentioned below.