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A founding father

Jack Dorsey is now once again the CEO of Twitter. He steps into the role after serving as interim chief since July 1, when then-CEO Dick Costolo stepped down.

Here's a brief history of Dorsey and the company that made 140-character messages hip.

Photo by: James Martin/CNET

@jack's first tweet

Dorsey, using the handle @jack, sent out his first tweet on March 21, 2006: "just setting up my twttr"

Dorsey conceived and created the microblogging site in 2006 along with Evan Williams and Biz Stone. Dorsey served as the first CEO of Twitter, which catapulted into the limelight in 2007 at the South by Southwest festival in Austin, Texas.

Photo by: Screenshot/Twitter

The Evan Williams era

In 2008, Dorsey traded roles with fellow Twitter co-founder Williams. Williams became CEO of Twitter, taking over its day-to-day operations, and Dorsey took on the role of chairman of the board. The move was seen as a signal that Twitter was serious about turning its rapid growth into revenue, something the tech company is still struggling to do today.

Williams (left) and Stone are seen here at Twitter's old office in the South of Market neighborhood of San Francisco. In 2012, Twitter moved into a new office on Market Street.

Photo by: Eric Luse/San Francisco Chronicle/Corbis

Tweet on the Hudson

In January 2009, a US Airways flight made an emergency landing in the Hudson River. All crew and passengers survived the incident, which came to be known as the "Miracle on the Hudson." One of the first photos from the scene was shared on Twitter, breaking the news before traditional media outlets.

The photo was taken and posted by Janis Krums, who was on a ferry rushing to rescue passengers huddled on the wings of the plane. He tweeted, "There's a plane in the Hudson. I'm on the ferry going to pick up the people. Crazy." The photo spread rapidly across the Web and crashed TwitPic, the third-party app that hosted the photo.

Photo by: @jkrums/Twitter

Dorsey founds Square

Dorsey again stepped into a role as CEO in 2009, this time at mobile-payments company Square.

Square is perhaps best known for the little white-plastic card-reader that plugs into a smartphone or tablet and enables small businesses to accept credit card payments. The company this month filed confidential paperwork with the Securities and Exchange Commission to sell stock to the public, according to several major news outlets, which cited unnamed sources.

As CEO of both companies, Dorsey may face conflicts of interest when allocating his time and expanding Twitter's role in e-commerce.

Photo by: Ramin Talaie/Corbis

Dick Costolo becomes CEO

In October 2010, Dick Costolo was promoted to CEO. He previously served at Twitter's chief operating officer and was instrumental in developing its fledgling business model. Williams stayed on at Twitter to focus on "product strategy," and he is currently still a member of Twitter's board. At the time Costolo took over, Twitter had 160 million registered users and roughly 90 million tweets were posted daily.

Photo by: Eric Gaillard/Reuters/Corbis

Arab Spring and 100 million milestone

Twitter played a role in what became known as the Arab Spring, a series of demonstrations and protest in 2011 across the Middle East and North Africa. Twitter was used to organize protests and rally support. It also allowed the world watch a revolution play out in real time.

Twitter also crossed several milestones in 2011. The company said in March 2011 that on average 1 billion tweets were sent every week. In September 2011, Twitter said it had 100 million monthly active users worldwide.

Photo by: Carlos Cazalis/Corbis

Twitter goes to the White House

Dorsey (left) moderated a "town hall" with President Barack Obama on July 6, 2011. People were able to ask the president questions via Twitter using the hashtag #AskObama.

Obama is known for his social media savvy and has managed to earn a few Twitter records.

After winning re-election in 2012, Obama's official Twitter account posted a photo of the president hugging First Lady Michelle Obama, with the message "Four more years." It quickly become the most retweeted message ever.

Obama set another record when he launched his own Twitter account, using the handle @POTUS, earlier this year. The account amassed 1 million followers in five hours, surpassing the mark set by "Iron Man" actor Robert Downey Jr., who attracted 1 million followers in just under 24 hours last year.

Caitlyn Jenner beat the president's record in June, netting 1 million followers in 4 hours and 3 minutes.

Photo by: Michael Reynolds/epa/Corbis

Twitter releases Vine

In January 2013, Twitter launched Vine. The app lets people create 6-second videos that loop indefinitely. The new service has proven its chops as a news-gathering tool, playing a role in the sharing of video during the Ferguson, Missouri, protests over police conduct in 2014.

Vine also joined forces with the world's largest short film festival, Tropfest, for a dedicated six-second film competition.

Photo by: James Martin/CNET

TWTR

Twitter became a publicly traded company on November 7, 2013. Twitter priced its shares at $26, but share rose 73 percent to $45.10 in the first day of trading.

Twitter shares closed at $26.31 on Friday ahead of the announcement of Dorsey's return.

Photo by: Andrew Gombert/epa/Corbis

Investors are unhappy

Despite Costolo's efforts to expand Twitter's appeal, investors have been unhappy.

In June, Twitter investor Chris Sacca voiced his concerns in an 8,500-word unapologetic blog post titled, "What Twitter Can Be." Sacca said that Twitter has disappointed Wall Street more often than not and that confidence in the management has diminished since it went public in 2013.

Sacca's comments foreshadowed Costolo's departure from the top job later in June.

Photo by: James Martin/CNET

Dorsey steps in -- on an interim basis

Costolo stepped down as CEO of Twitter at the end of June but at that time remained on the company's board of directors. With Dorsey's official return as CEO, Costolo has resigned from the board.

"Despite all that we have accomplished, Twitter still has huge unmet potential," Dorsey said in June.

Dorsey served as interim CEO for three months.

Photo by: James Martin/CNET

Snowden pops up on Twitter

On Tuesday, Dorsey welcomed to the social network new tweeter Edward Snowden, who stole and leaked thousands of National Security Agency documents when he worked there years ago as a contractor.

Hours after Snowden joined Twitter, Republican presidential candidate and former New York Gov. George Pataki called on the site to shut down Snowden's account.

Photo by: Screenshot/Twitter

Two jobs

Dorsey now has the rare position in the tech industry of running two companies simultaneously.

Steve Jobs and Elon Musk are among the few other chief executives to oversee two companies at the same time. Jobs ran Apple while heading Pixar; Musk is CEO of both Tesla Motors and SpaceX.

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