Even as some naysayers have questioned Twitter’s business model, the microblogging site has landed the largest venture capital investment in history as part of an $800 million funding deal.

Although Twitter would not confirm the size of the round led by Russian venture firm DST Global, saying only that it was “significant,” regulatory documents indicate the company has sold $400 million in new shares. That likely values the 4-year-old startup at $8 billion, more than double its worth after closing its last venture round in December.

Accompanying the $400 million investment, according to a person close to the company, is another $400 million to let Twitter’s employees and investors cash out some of their shares.

DST used the same tactic two years ago in the transaction that put it on Silicon Valley’s map: A $400 million investment in Facebook. Half of that money went to the company, and half to give insiders liquidity while easing pressure on management to go public.

San Francisco-based Twitter said some of its prior investors participated with DST in the funding round, the company’s seventh.

Documents from the secretary of state’s office in Delaware, where Twitter is incorporated, indicate that the company agreed to sell about 24.9 million shares of preferred stock, priced at $16.09 apiece, according to VC Experts, a data provider for the VC industry.

Justin Byers, head of business intelligence for VC Experts, said that stock is divided into two classes, only one of which grants the right to participate in the election of Twitter’s board members. Byers noted that in past investments in companies like Groupon and Zynga, DST has declined to take board seats, “so it’ll be interesting to see if they did somehow get a seat out of this.”

In a statement released by Twitter, DST Global founder Yuri Milner called the startup “one of the few companies that has truly changed the world.” He added, “We couldn’t pass up the chance to be a bigger part of its future.”

Byers said regulatory filings indicate Twitter has authorized for sale 600 million shares of common stock. Given the price of the shares authorized in the new agreement, “It’s very possible they’d be at an $8 billion valuation.”

That’s 40 times the $200 million in yearly revenues that equity research firm Hudson Square recently estimated Twitter takes in.

Byers said there are no regulatory filings concerning the reported $400 million side deal by DST, since that agreement would cover existing stock from shareholders rather than new shares issued by Twitter.

Even the $400 million injection the company itself is receiving breaks the record for venture capital deals. “There’s nothing else bigger out there,” said John Taylor, research chief for Virginia-based National Venture Capital Association.

In fact, he said, only four companies have ever received more than $400 million in total venture funding, much less that amount in one fell swoop. According to CrunchBase, Twitter had taken in $360 million in venture capital before Monday.

Twitter in a statement said it would use the money “to aggressively innovate, hire more great people and invest in international expansion.” It noted that in the past year, its head count has grown to 600, and it’s more than tripled the number of tweets transmitted each day, to more than 200 million.

Contact Peter Delevett at 408-271-3638.