Web spam is more viral than email spam.
In email spam they target only known email id, which they get from various sources like small consultancies or from training institutes or by doing some social engineering ;). Mail providers like Gmail, Yahoo are too intelligent to send it to spam folder without giving you trouble. So, you are somehow safe in case of email spam.
But in case of web spam, they want us to browse their website to buy or advertise their products. Web Spammers main motto is only to earn money. We need to be sure before any comment or post is getting published in our website. In worst case you may lose permanent genuine readers too!, which will ultimately affect your site and your revenue if any.