Some our our sites that feature free classified ads (amadorable.com, goatseeker.com, and bunnytrade.com) have been hit with a few recurring spammers trying to plant ads for various off-topic products (like cell phones, etc.) I guess this is a good sign; our sites are visible and spammers feel it is worth their time and trouble to post an ad (and no, as far as I can tell, these are not bots - they're human-generated spam.)
In reviewing the referrer logs, I've noticed that in nearly every case, spammers use search engines to find sites that have been spammed previously using known keywords - or just sites that offer free classified ads or open posting capability. For example, I find these google searches in my logs, just prior to the spammer creating an account and attempting to deposit the spam content.
