Rumors, according researchers, aren't just idle or malicious gossips: instead, they are driven by real curiosity and the desire for knowledge. Even negative rumors often serve as a "glue" for social networks (in real life, and online):
Aside from their use as a news grapevine, rumors serve a second purpose as well, researchers have found: People spread them to shore up their social networks, and boost their own importance within them. To the extent people do have an agenda in spreading rumors, it's directed more at the people they're spreading them to, rather than at the subject of the rumor.
People are rather specific about which rumors they share, and with whom, researchers have found: They tend to spread rumors to warn friends of potential trouble, or otherwise help them, while remaining mum if it would be harmful to spread a given rumor in a certain context or to a certain person.
That's why the best way to combat rumors isn't to deny that they exist, instead, the best way to stem a rumor is actually to spread it:
Other than denying a rumor that's true, perhaps the biggest mistake one can make, DiFonzo and other researchers say, is to adopt a "no comment" policy: Numerous studies have shown that rumors thrive in environments of uncertainty. Considering that rumors often represent a real attempt to get at the truth, the best way to fight them is to address them in as comprehensive a manner as possible.
Anthony Pratkanis, a psychologist at the University of California, Santa Cruz, who studies persuasion and propaganda, says that an effective rebuttal will be more than a denial - it will create a new truth, including an explanation of why the rumor exists and who is benefiting from it.
Here's an interesting article by Jesse Singal for The Boston Globe (note: the article does use how Obama and McCain campaigns deal with rumors to illustrate ways to deal with rumors, but I think the post merits a thoughtful reading sans the political connection): Link