Chinese e-commerce giant Alibaba Group is the most valuable private tech company in the world, topping better-known privateers such as Twitter and Bloomberg, according to Business Insider.

The business and financial news website estimated Alibaba Group is worth about $40 billion, based on a valuation established by Yahoo’s recent sale of half its stake in Alibaba. Finishing a close second on Business Insider’s annual “THE DIGITAL 100: The World’s Most Valuable Private Tech Companies” list is Bloomberg, the financial information services company, worth an estimated $35 billion.

From there the valuations plummet. No. 3 Twitter, the microblogging/social networking service, is worth just $5.25 billion, according to BI estimates, while No. 4, Chinese retail website 360Buy, is worth $5 billion.

Financial results for Alibaba Group are hard to come by because (of course) private companies don’t have to disclose them. But the company’s major assets, which include China’s largest e-commerce websites Taobao Marketplace and Tmall, have been growing very quickly due to the explosive growth of online spending by Chinese consumers.

In the first quarter of 2012, the group had a net profit of $235 million, up 356% year-on-year, on sales of$805 million, up 64% year-on-year, according to a report Yahoo filed with the U.S. Securities and Exchange Commission earlier this year. Taobao and Tmall are expected to host a trillion yuan ($157 billion) in transactions this year, up 150% from 400 billion yuan in 2010.

A little perspective: Alibaba Group may well be the world’s most valuable private tech company. But the most valuable tech company of all is Apple, with a market cap ofabout $625 billion, 15 times the size of Alibaba.