It means "Fear, Uncertainty, Doubt".

It's a modern kind of shit talking employed by large corporations in conjunction with astro turfing to dissuade people from going to a competitor.

Because saying stuff like "I dunno dude, there's been reports of a lot of problems with Product X" is a lot more persuasive than "Product X sux!!!", especially when you don't know the poster was paid by Company Y, a competitor, to say it.