Here's an article from emarketer on this topic:
Of all word-of-mouth (WOM) tactics, viral marketing has probably drawn the most attention from marketers. And they have lofty goals for it. According to a 2006 study by JupiterResearch, cited in Internet Retailer, the biggest goal of viral marketers was to increase brand awareness (71%). Half also expected to drive online sales, and 44% hoped to drive offline sales.
B2B and B2C marketers diverge on what they consider successful viral tactics. B2C companies were more likely to consider blog mentions and posting to sites such as MySpace or YouTube to be something that would generate attention for their viral campaigns. B2B marketers were more interested in getting their viral campaigns mentioned in an online publication or business print publication, according to MarketingSherpa.
Consumers often learn about videos from their friends. According to a 2007 survey by the Online Publishers Association, 43% of US online video viewers get videos by clicking links in e-mail messages from people they know.