Economics Report.Facebook is the world'sbiggest social network-- and the subjectof the movie "Social Network."(SOUND)The real Mark Zuckerbergand his friends at Harvard Universitylaunched the site in two thousand four.Facebook says it reachedfive hundred million users last July.Now, the American bank Goldman Sachsand the Russian companyDigital Sky Technologieshave friended Facebook.They are investing a totalof five hundred million dollarsin the company.The deal values Facebookat fifty billion dollars-- more than many publiclytraded Internet companies.Goldman Sachs is expected to raisea billion and a half dollars moreby selling shares of ownershipin Facebook to rich investors.The plan does not includea public stock offering-- at least not right now.For now, Facebook would remaina private company-- meaning a company that does notsell shares to the public.The plan has brought new attentionto the largely secretive worldof private financing and the rulesfor private companiesin the United States.The idea is that investorsin public companieshave protections that investorsin private companies do not.The Securities and Exchange Commission saysa private company must reportfinancial information if it hasfive hundred shareholders or more.A new business, a startup company,is usually considered too riskyfor average investors.But a promising startup may finda small number of private investors,often known as "angels."These investors are willingto lose everythingfor a chance at big returns.Rikki Tahta has been involvedin raising money for startups.He is now chairman of his owninvestment company, Covestor,with offices in New York and London.Mr. Tahta compares the differencebetween public and private companiesto the differencebetween marriage and dating.When people are dating, he says,there are understandingsbut few rules.In marriage, the rulesare more clear and well-defined.In his opinion, the only real benefitfor a private company is loweradministrative and record-keeping costs.Yet he tells us Covestor remainsa private company after a few yearsbecause it is still too riskyfor most investors.And that's the VOA Special EnglishEconomics Report,written by Mario Ritter.You can comment on our programsand find transcripts and MP3sat 51voa.com.We're on Facebook, Twitterand YouTube at VOA Learning English.