Heating up the ongoing social media war for online advertising, Twitter has rejigged its top guns to take on Facebook. Co-founder Jack Dorsey, whose first ever tweet was recently recalled on the occasion of the site's fifth birth anniversary, has returned to work at Twitter, where his forte in product development is expected to help the microblogging site beat the social networking rival in online advertising.

Dorsey, the erstwhile CEO who stepped down and turned chairman in 2008, tweeted announcing his return to Twitter: I'm thrilled to get back to work at @Twitter leading product as Executive Chairman.

He also informed that he would be dividing his time between Twitter and his other start-up, Square, adding: And yes: leading @Square forevermore as CEO.

Although his decision to split time between the two roles has raised a few questions, Dorsey's return to Twitter is expected to bolster the company's fight to grab the majority hold on online advertising. Dorsey is now the head of product and his title has been changed from Twitter's chairman to executive chairman.

Dorsey comes in at a time when Twitter's war with Facebook on the revenue front was heating up. According to eMarketer Inc, Facebook, which is two years older than Twitter, is expected to make up 42 percent of the overall U.S. population this year, up from 38 percent last year, while the microblogging site it has more than 200 million registered accounts with almost 500,000 accounts adding everyday. The research firm also estimated that Twitter will bring in advertising revenue of about $150 million this year.

As the nature of the two sites account for the varying time that users spend on them, Twitter is now aiming to get users to spend more time on the site.

Quoting Debra Aho Williamson, an analyst at EMarketer Inc, San Francisco Chronicle reported that Twitter is expected to generate less than one-tenth the advertising sales of Facebook this year and its user base is expanding more slowly.

Twitter, therefore, needs Dorsey to develop other products that get users to spend more time on the site.

To be anywhere close to the success that Facebook has achieved, Twitter just needs to be used by more people on a more regular basis, Williamson is quoted as saying.

The plan is simple: create interesting products to ensure users stay on the site longer, translating into surge in popularity and consequently revenue.

While Facebook brought in ad revenue of $1.86 billion last year, eMarketer forecasts that Twitter may more than triple to about $150 million this year.

Meanwhile, with the return of Dorsey come questions on the role of co-founder Evan Williams, who handed over the CEO reigns to Dick Costolo last October to focus on product development.

The two co-founders have a contentious relationship, TechCrunch noted in a report on shedding light on how Evan Williams has become less involved at Twitter over the months.

A company spokesperson told the outlet, Ev decided a couple months ago to be less involved day to day at Twitter. He continues to have a close relationship with the company providing strategic advice and, of course, he remains an active board member.