Did GM just pop the Facebook bubble? Though the announcement that the auto giant will pull its paid advertising from Facebook is unlikely to seriously undermine the latter's upcoming IPO, it surely will cast a shadow over the much-hyped event. 

The company's short lifespan and quixotic management have already generated plenty of skepticism about the pricing of the shares, which value the company at more than $100 billion -- or roughly the size of Morocco. 

Now its basic business model is under attack.

GM's news raises two glaring issues confronting would-be investors. 

First, the auto maker reports that its $10 million of paid Facebook ads haven't generated sales.

Second, the company says it will continue to have a presence on Facebook -- by hosting pages which are free. 

That its advertising has been ineffective is alarming; that the country's third-largest advertiser