No surprise that today’s announcement came in a Tweet: “We’ve confidentially submitted an S-1 to the SEC for a planned IPO. This Tweet does not constitute an offer of any securities for sale.” A recent rule change at the SEC allows companies with annual revenues of less than $1B to make a confidential filing. That gives them an opportunity to clear things with the regulators before rolling out detailed plans to the public. CNBC says that Goldman Sachs will be the lead underwriter, and that Twitter could be valued at as much as $15B. The valuation will be key following the fiasco that took place last year after Facebook went public. It sold shares for $38 apiece, but the price quickly plummeted as investors saw indications that the company might lose advertisers as Facebook users increasingly accessed the social network on smartphones. The company has assuaged those fears in recent months and the stock recovered, closing Thursday at $44.75.