With its mammoth collection of more than 500 million members, why wouldn’t Facebook enter the group buying world? People are already on Facebook and engaged with what their friends are doing plus group buying is the ‘it’ thing now.

So Facebook will soon begin testing its service that offers members local discounts via it’s Facebook Places when people check into local businesses. The service will start testing in Dallas, Austin, Atalanta, San Franciso and San Diego and will expand through the Deals program.

Facebook will leverage its existing sales force to source deals and will partner with a number of partners as well.

Facebook recorded US$873 million in revenue last year but could hit US$3.93 billion by 2015, according to estimates from consulting firm BIA/Kelsey. These numbers are making institutional investors excited, buying shares from Facebook employees pushing its valuation to around US$50 billion. There are rumors flying around that Groupon, the most prominent group buying site is valued at US$25 billion as it plans for IPO. Google will also be launching a social buying service called Google Offers.