A survey by ComScore in conjunction with Facebook looks closely at what the potential reach of not only the “fans” but “friends of fans” . This what you call the ‘secondary effect’. These friends of fans represent a much larger set of consumers (34 times larger, on average, for the top 100 brand pages)

While Bing with a fan base of approximately 1.7 million fans on Facebook, the number of “Friends of Fans” is 232 million – more than 130 times the size of its Fan base

via http://www.jeffbullas.com/2011/07/28/is-this-the-holy-grail-of-facebook-marketing/