According to sources who told Bloomberg, co-founder Evan Williams would be Twitter Inc.'s biggest shareholder with a 15% ownership. The sources also added that Williams has a bigger holding than co-founder Jack Dorsey. The sources asked not to be identified as ownership details had not been made public. 

Williams' position in the company would reportedly make him a billionaire after the company's initial public offering. Bloomberg reported that should Twitter's market capitalization surpass the projected USD10.5 billion market valuation, Williams' holding would be worth USD1.5 billion. Partner Maha Ibrahim of venture firm Canaan Partners said, "It's a testament to his belief in the company that he's held on as long as he has. There's been so much opportunity to sell on the secondary market."

When asked for comment, Williams nor spokesman Gabriel Stricker returned a response to Bloomberg.

Aside from Williams and Dorsey, Twitter shareholders whose stake would be valued in the USD1 billion range would be investment firm Union Square Ventures. Union Square partner Fred Wilson led the first financial round of Twitter back in 2007.