While it doesn't take much investment to send out massive amounts of emails, the potential for being fined for sending spam seems like it should be some sort of deterrent for an act which, as near as I can tell, has little benefit to it.
I can understand the point of message board spam, because that's actually far less about getting the posters on the board to look at the links and far more about hiding the links so that they'll be picked up by search engine spiders and raise your site's rank.
But email or instant messenger spam seem utterly pointless to me. If some spammer manages to get into my inbox, my response isn't, "Oh boy, a webcam with live nudes!" It's closer to, "Goddamnit. I thought this filter caught all of these." So are there enough people out there who can be swayed by spam that it continues to be lucrative? Do some people lose all sense or reason when they see that they can buy herbal Viagra for CHEAP CHEAP CHEAP?