As far as smaller imageboards are concerned, Grimchan was pretty big in its hayday, usually about 600 visitors or so per month. Spam was generally posted several times per day, mostly late at night/early in the morning.
As of now, the URL isn't in any of the imageboard list pastebins people insist on making so bots haven't found us yet. Been mostly smooth since the URL switch, but that will certainly change.
As I've said before, the best way to advertise is to tell people yourself. I can't afford to pay banner ads, I barely made the server payment this year lul.