Virtual goods are fast becoming big business for real.

Surging online sales of digital goods — everything from digital pets to “currency” for online game-playing to virtual roses for a love interest — is driving growth and reshaping business plans among Internet companies small and large, particularly around video gaming, offering new revenue streams as web advertising rates have fallen. The trend is fueling growth and innovation among game publishers, dating sites, advertising agencies, payment processing companies and purveyors of industry analytics.

For now, the mac daddy of North American virtual goods is Zynga, the Potrero Hill, California, online social gaming company founded in 2007, which gets two-thirds of its projected $100 million-plus annual revenue selling virtual goods ranging from digital farm buildings to poker chips.

In 2009, an estimated $400 million to $600 million will be spent on virtual goods in the United States, and $5.5 billion globally, according to Brian Balfour, founder of Viximo. His Cambridge, Massachusetts-based firm provides virtual goods services to social networking, online dating and casual gaming sites. U.S. virtual goods spending was likely less than half that in 2008, and between $25 million and $50 million in 2007, Balfour said.

The growth rate is expected to increase. The No. 1 social network, Palo Alto, California-based Facebook, is now testing its own virtual currency, something that another social network, Hi5, has successfully deployed and multiple other social networks are rumored to be considering. In March, Apple also officially announced that microtransactions would be possible in iPhone applications, setting the stage for a massive explosion of virtual goods sales on the world’s most popular smartphone platform.

At Zynga, one-third of its revenue stream comes from direct-to-consumer sales of virtual goods, another third comes from advertiser-sponsored offers in which users can get virtual currency to buy virtual goods, and another third comes from traditional advertising. Zynga has been expanding hugely, rising from 263 full-time employees just in June to its current 340 people, and 110 positions remain open for hiring.

Virtual goods and virtual currencies were pioneered in Asia, particularly in China and South Korea, where free games with premium content proved popular. One Chinese company, Tencent, does nearly $1 billion a year in virtual goods sales by itself.

In San Francisco, Second Life maker Linden Lab in 2003 launched a pioneering virtual economy with its Linden Dollars, which are exchangeable for U.S. dollars, but other than leasing virtual land the company lets others do the trading and does not sell virtual goods. Also in San Francisco, dating site HotorNot started selling digital roses for $2 to $10 in 2004.

Zynga CEO Mark Pincus believes North American virtual goods activity could exceed $15 billion by 2014, and he says, “San Francisco has become the Motor City for virtual goods and social games.”

Numerous industry experts said U.S. virtual economic activity escalated dramatically in the last year as the economy suffered, ad rates dropped dramatically and publishers went looking for alternative sources of revenue.