Facebook’s first-half revenue roughly doubled to $1.6 billion, underscoring the world’s largest social network’s appeal to advertisers, a source with knowledge of its financials told Reuters. Net income in the first half of 2011 came to almost $500 million, said the source, who wished to remain anonymous because privately-held Facebook does not disclose its results. Facebook’s stronger results come as investors have pushed its valuation to roughly $80 billion in private markets, with many industry observers expecting the world’s No. 1 Internet social network to go public in 2012.