Fast growing startup Twitter will soon be joining a select group of startups with private venture round valuations of $1 billion, we’ve heard from multiple sources. CEO Evan Williams disclosed the round to employees at a recent all hands meeting.

The company will raise around $50 million, we’ve heard, although the final amount of the raise is apparently not yet locked down.

Twitter raised $35+ million earlier this year in a round led by Benchmark Capital and Institutional Venture Partners. That round valued the company at $250 million.

The company has raised a total of around $55 million to date, and sources tell us they have approximately $30 million left in the bank.

Update: A source tells us that New York based Insight Venture Partners is the primary investor in this round, and that the company may raise more than $50 million.