Doing some analysis over three Pages (The Contender Australia, Grinspoon, Vanessa Amorosi) I am currently working on I worked out that for every three people that join a Page through paid media, they will indirectly influence two friends within their network to join the Page via the suggestion box/highlight or news feeds of their friends pages.
In my opinion Facebook is the only social platform that has an advertising model that is worth investing in. We are seeing some of the smart agencies in Australia pick up on this and really launch some big exciting campaigns through the platform Telstra Digital Mums (DDB), The Skinny Cow (Publicis Digital), 5 Seeds (Holler), and Lynx (SOAP).
Methodology: Since the upgrade to Facebook Ads in June 2009, they now give you action rates which is how many people become a fan directly from the ad. So subtracting this action rate number from your Page growth gives you the viral growth rate, I then made that number into a percentage (Grinspoon - 34%, Vanessa Amorosi - 39% and The Contender - 40%). I would love it if anyone else would like to share their stats