Two of Twitter's earliest investors have left the company's board of directors, the latest change to the fast-growing social media company's leadership.

Fred Wilson of Union Square Ventures and Bijan Sabet of Spark Capital are no longer on Twitter's board, the company said on Friday.

It was not immediately clear what prompted the change.

Twitter does not plan to appoint new directors to replace Wilson and Sabet, a source familiar with the matter said.

Wilson and Sabet did not return emails seeking comment.

The departures mark the latest change to Twitter, whose microblogging service allows users to broadcast short, 140-character messages, or tweets, to groups of followers.

In October, Dick Costolo replaced co-founder Evan Williams as chief executive. Earlier this year Costolo brought Jack Dorsey, another co-founder, back to the company as executive chairman.

Williams and Twitter co-founder Biz Stone have recently started a new project called Obvious Corp.

Twitter, which has more than 100 million active users, has been increasingly focused on building a revenue-generating advertising business on top of its Web service.

The company is broadening the areas of its service where ads will appear, Costolo said during a briefing with reporters and analysts at the company's San Francisco headquarters last week.

During the meeting, Costolo repeatedly cited Wilson in examples and anecdotes about the various uses for Twitter, at one point using the example of Wilson's son and his proclivity for following NBA players on Twitter without actually contributing any tweets himself.

Twitter, which raised $400 million in a new round of funding earlier this year, credited Wilson and Sabet on Friday with playing important roles in the company's success.

Both saw what Twitter could become before most anyone else, said Twitter spokesman Sean Garrett in an emailed statement. We look forward to their continued input as both investors in the company and passionate users of the product.

The departures were first reported on the technology blog AllThingsD.com.

(Reporting by Alexei Oreskovic in San Francisco; Editing by Phil Berlowitz)

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