It's a particular plague on the free classified sites and will kill them off. This includes the personals sites where many people are looking for someone to meet. As soon as the spammers can infiltrate the pay sites, they will inundate them too. Most of the spam that shows up in mailboxes and newsgroups and classifieds is promoting either sex sites or multilevel marketing schemes. Much of it is hard to stop, although not impossible.
Filters do eliminate spam mail.
Yahoo classifieds, for example, are riddled with spam. The personals are a particular embarrassment. It's time users rebelled against this epidemic to make sites stop selling ads all over the page. A nice banner across the top is the way all this got started. It was touch and go at first.