Wherever you might expect to find information, spammers will be eager to insert some disinformation.
Consider the various online reputation systems–for example eBay’s feedback system or just about anyone’s list of popular bloggers.
Positive spam attempts to raise reputation scores in some deceptive way–maybe creating a bunch of fake eBay transactions, then giving yourself very positive ratings.
Negative spam has a much higher cost to the commons. Trolls who take pleasure in “gratuitous vituperation, scurrilous disparagement and malignant calumny,” (to quote the online Britannica on defamation) drive productive group members away from the commons.