Online advertising, also called online marketing or Internet advertising or web advertising, is a form of marketing and advertising which uses the Internet to deliver promotional marketing messages to consumers. Consumers view online advertising as an unwanted distraction with few benefits and have increasingly turned to ad blocking for a variety of reasons. When software is used to do the purchasing, it is known as programmatic advertising.
Dropbox was at least partially inspired by PayPal, which literally paid people money (through the PayPal system) if they got their friends to sign up. They did this because they benefited from the network effects of having large number of users- so even if half of the signups were people who just wanted the free money, the end result is a large number of people having PayPal accounts- which is a useful position for a payments company to be in.
According to a paper by Duncan Watts and colleagues entitled: "Everyone's an influencer", the most common risk in viral marketing is that of the influencer not passing on the message, which can lead to the failure of the viral marketing campaign. A second risk is that the influencer modifies the content of the message. A third risk is that influencers pass on the wrong message. This can result from a misunderstanding or as a deliberate move.