It looks like Facebook's market valuation does have a limit after all. Early-stage investors and employees of the social network giant are trying to unload $1 billion of Facebook shares citing concerns that the site's growth can't keep up with its sky high valuations, Reuters reports. The news service cites five sources "with direct knowledge of the situation" who say the group had tried unloading its shares at a $90 billion valuation but there weren't any takers so they're now willing to settle for $70 billion. The article also gets some notable investors on the record saying the rising valuations have gotten out of hand. "At the current valuation where it is, it is really hard to justify the investment," says Sumeet Jain of CMEA Capital who passed on investing in Facebook. "It's hard to imagine it will turn into a $270 billion company in the next few years."

No one questions Facebook's explosive popularity, which boasts over 600 million members and more page views than Google. But will it be able to monetize its traffic as effectively as Google? Up until now valuations of the company have only risen sharply. Here's how far the company has come in the last 6 years. In 2005 Facebook raised $12.7 million from Accel Partners in a valuation rumored at $100 million. In 2006 $27.5 million from Greylock Partners and Meritech Capital gave it a $525 million valuation. In 2007, Microsoft got the ball rolling with a $240 million investment into Facebook valuing it at $15 billion. In 2009 Digital Sky Technologies put a slight dent in the company's valuation, investing $200 million at a valuation of $10 billion. In 2010, Facebook was trading at more than $16 per share on SecondMarket, pegging its estimated value at $41 billion, making it the third biggest web company. In January 2011, a $500 million investment from Goldman Sachs and Digital Sky Technologies valued the company at $50 billion. Not too shabby for a company that hauled $355 million in net income in the last nine months on revenue of $1.2 billion.

Too see what that kind of metoric growth looks like on a chart, here's a quick sketch: