Exclusive: Elon Musk quits Zuckerberg's immigration advocacy group | Reuters

Musk leads one of the world's best known "green" companies, electric carmaker Tesla. A Tesla spokeswoman told Reuters on Friday that the South African-born entrepreneur preferred not to elaborate on his reasons for leaving FWD.us.

Zuckerberg announced the formation of FWD.us last month, saying it was focused on bipartisan policies to bring about comprehensive immigration reform and improvements in the U.S. education system.

Fwd.us bankrolled three television ads on behalf of senators who have been playing a key role in the immigration debate.

The ads were focused not on immigration but rather on the senator's general positions, including one's support for the Keystone XL pipeline, which has created a backlash among some progressive groups. Backers say the pipeline project would boost North American energy security and provide thousands of construction jobs. Opponents argue that it would lead to higher releases of greenhouse gases.

The spots quickly drew the ire of liberal and environmental groups, including the Sierra Club and MoveOn.org, who earlier this week pledged to pull ads from Facebook for two weeks.

The Fwd.us website removed Musk's name on Friday after a Reuters inquiry. It was unclear how much Musk, who also chairs solar-energy company Solar City, had donated.

David Sacks, founder of business networking site Yammer and a former colleague of Musk's at payment service PayPal, also dropped off the list of the FWD.us backers on Friday. FWD.us spokeswoman Kate Hansen earlier confirmed that a second funder had withdrawn support but declined to elaborate. Sacks did not immediately respond to a request for comment.

"We recognize that not everyone will always agree with or be pleased by our strategy - and we're grateful for the continued support of our dedicated founders and major contributors," FWD.us spokeswoman Hansen said in a statement.

"FWD.us remains totally committed to supporting a bipartisan policy agenda that will boost the knowledge economy, including comprehensive immigration reform."

A Facebook spokeswoman did not immediately respond to a request for comment.

A DIFFERENT APPROACH

FWD.us boasts an impressive list of backers, including Microsoft founder Bill Gates and LinkedIn founder Reid Hoffman, though it operates as a special type of non-profit group that does not have to disclose its donors. Run by Zuckerberg's old Harvard roommate, Joe Green, FWD.us's political operation is managed by a group of Washington insiders with leadership roles in both Republican and Democratic organizations.

It joins other technology groups and alliances lobbying Congress for more H-1B visas for high-skilled workers and easier hiring of foreign math, science and engineering experts.

Rather than directly representing the companies its backers are involved with, FWD.us is funded by individuals who have personally attached themselves - and their cash - to the cause. Zuckerberg has become the group's public face, among more than three dozen big-name supporters.

Politically, Zuckerberg has carved out bipartisan credentials, visiting the White House and hosting a town hall for President Barack Obama but also staging a fundraiser for Chris Christie, the Republican governor of New Jersey who some see as a 2016 Presidential hopeful.

The latest controversy was sparked by ads for Republican Senators Marco Rubio from Florida and Lindsey Graham from South Carolina as well as Alaskan Democrat Mark Begich. FWD.us helped fund two separate entities to run the ads.

Rather than focusing on raising public awareness over immigration reform and its benefits for the technology sector, the FWD.us ads promote lawmakers who the groups thinks will be key players on the issue.

In doing so, the ads highlight a number of positions held by the senators, including supporting the controversial Keystone XL pipeline in Graham's case, and drilling in the Alaska National Wildlife Preserve in Begich's.

The ads could help inoculate the lawmakers against challenges from within their parties as a result of their stand on immigration reform. Graham and Begich are up for re-election in 2014.

Fwd.us co-founder Jim Breyer, a venture capitalist at Accel Partners, defended the group's efforts.

"Our advertising decisions are being made by a very smart team of political operatives who know that passing major reform will require some different and innovative tactics," Breyer said in an emailed statement.

For the most part, donors have stayed out of the nitty gritty of how FWD.us operates.

"It's a really gnarly, gnarly thing having to deal with Washington," venture investor and Fwd.us co-founder Chamath Palihapitiya said at a conference last month. "I'm glad that other people other than me are dealing with it who have the patience and the resolve to figure it out."

(Reporting By Sarah McBride and Alina Selyukh; Editing by Ros Krasny, Mary Milliken and Claudia Parsons)