Twitter has filed for an IPO, it announced on its own Twitter feed.

The S-1 filing has been long-awaited. It will give us a first glimpse of the real financial numbers behind the micro-blogging company. S-1 IPO filings with the SEC require that the company disclose its historic and current revenues, profits and balance sheet holdings, prior to selling stock to the public.

Here's the tweet:

Twitter

In what appears to be a joke, Twitter immediately followed its announcement with this tweet (below).

Twitter The IPO filing had been expected. Twitter has clearly grown into a huge business — it's expected to do more than $500 million in revenues this year and has taken more than $1 billion in funds from investors — and the rumors and chatter swirling around the company had reached fever pitch.

We noted earlier today that Wall Street and other observers all seemed to be behaving as if an IPO was expected.

The most recent tantalizing clue was the fact that Twitter CEO Dick Costolo declined to answer questions yesterday after giving a speech on leadership at Techcrunch Disrupt. Companies that are filing paperwork with the SEC often go through "quiet periods" which restrict executives from saying anything that might prime the market.

This post tells you the background to Twitter's IPO filing.

Months after becoming Twitter's new finance chief, Mike Gupta began talking to investment banks about handling the company's initial public offering, according to the San Francisco Chronicle. Gupta has a previous relationship with Morgan Stanley and Goldman Sachs, with whom he worked at Zynga. Jared Keller, a freelance reporter, says Goldman is leading the underwriting.

The IPO filing is currently "confidential," according to the tweet. Confidential IPOs are favored by companies that have less than $1 billion in annual revenue:

Filing as an emerging-growth company requires less paperwork, including submitting just two years (rather than the usual three) of financial statements to the SEC. What's more: Every bit of the filing, including correspondence about it, is kept out of the eyes of the media, competitors, and the investment community until the company decides to make it public (no later than 21 days before the IPO presentation series, known as a roadshow).

MarketWatch has a good explanation also:

The confidential nature of the filing appears to be based on the law passed last year, called Jumpstart Our Business Startups Act, or JOBS Act, which allows some companies planning to go public to keep some of its information confidential, until the deal gets closer to pricing.

Some believed Twitter had held off filing for an IPO after the experience of Facebook, which went public at $38 but then immediately became embroiled in a controversy over whether Wall Street analysts had been given different information in phone calls with the company than was contained in the company's other disclosures.

Retail investors interest in a Twitter filing will likely be intense: Although Facebook's stock initially fell in value by half it has since popped up to $44.75. Facebook's investor relations crew has basically spent the last year explaining to investors how social media advertising and marketing works — and Twitter will benefit from that educated investor market.

P.S. Note that Business Insider was just 12 days amiss in its February estimate of when Twitter would file for an IPO.