Pandora and Groupon have filed for their Initial Public Offerings (IPOs) and social games firm Zynga is rumoured to be next. And Facebook? There are new rumours that the social network is set to go public, possibly by the first quarter of 2012 with a whopping valuation of $100 billion. CNBC has the story, claiming that the IPO will be triggered by Facebook passing the 500 investors milestone – a point at which it would have to begin publishing quarterly financials anyway, under SEC rules in the US. Chief operating officer Sheryl Sandberg has described an IPO for the company as “inevitable… the next thing that happens”.