SAN FRANCISCO — In June 2007, Evan Williams was looking for investors for a quirky Internet communications service called Twitter that he had co-founded.

He had already signed up a number of well-known Silicon Valley financiers, but he also dashed off a note to his old friend Dick Costolo, who had just sold his company to Google, asking if he would like to put in $25,000 or $100,000.

“I’m on the $25k bus,” Mr. Costolo replied three minutes after receiving the e-mail. “Thanks Ev, this will be a lot of fun.”

Mr. Costolo, who is now the chief executive of Twitter, is one of a handful of individual investors who stand to reap the rewards of a potential initial public offering of stock in the social network. The company said on Thursday that it had filed early paperwork with regulators to conduct such a sale, which will probably occur late this year or early next year.