Facebook's first-half revenue roughly doubled to $1.6 billion, underscoring the world's largest social network's appeal to advertisers.

Net income in the first half of 2011 came to almost $500 million, according to a source, who wished to remain anonymous because privately-held Facebook does not disclose its results.

Facebook's strengthening results come as investors have pushed the company's valuation to roughly $80 billion in secondary markets, with many industry observers expecting the company to go public in 2012.

With more than 750 million users, Facebook has become one of the Web's most visited destinations by consumers who spend hours on the site every month sharing photos and videos, and conversing with friends.

Its popularity among consumers and advertisers has pressured entrenched Web companies such as Yahoo Inc, whose board of directors fired CEO Carol Bartz on Tuesday for failing to revive the company's fortunes.

But competition is heating up. Search leader Google launched a rival social network in June that attracted more than 10 million users in its first two weeks.

Facebook declined to comment on its financial results.

(Editing by Edwin Chan and Derek Caney)

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