Facebook and Google Inc are separately considering a tie-up with Skype after the web video conferencing service delayed its initial public offering, two sources with direct knowledge of the discussions told Reuters .

Facebook Chief Executive Mark Zuckerberg has taken part in internal discussions about buying Skype, according to one of the sources. Another source said Facebook had reached out to the Luxembourg-based company about forming a joint venture.

Google has also held early talks for a joint venture with Skype, the second source said.

A Skype deal could be valued at $3 billion to $4 billion, the first source said. Skype's IPO is expected to raise about $1 billion, several other sources said.

The discussions are in early stages, and it is not clear which option the companies favor, the first two sources said.

Although an IPO is still in the cards for the second half of 2011, Skype remains in discussions with other companies, two of the sources said. If it goes through, a Skype IPO would be one of the most hotly anticipated debuts by a U.S. technology company this year.

Securing Skype as a partner would expand Facebook's user base, help it grow in international markets where Skype is popular, and give its half-billion users another reason to remain active and connected to its online community.

Analysts say a tie-up between Facebook and Skype would make more sense than one with Google, which already has a similar service - Google Voice.

Skype and Google declined to comment. Facebook was not immediately available to comment. The information is not public and the sources declined to be named.

GOOD TIMING

With a partnership, Facebook can tack another service onto its ever-expanding menu -- a crucial feature given that many mobile devices, including tablets, now come equipped with front-facing cameras.

"This is very synergistic," said Trip Chowdhry, an analyst with Global Equities Research. "It puts Facebook two steps ahead of Google because of the number of Skype users."

"In your social network, you will now have another very compelling service -- Skype," he added.

Last year, Skype had about 124 million connected users every month by the end of June. But 8.1 million were paying customers, using Skype to make calls to traditional phones at discounted rates.

Analysts have said that while Skype's growth has been impressive, investors would be cautious about its prospects for revenue growth due to the size of its base of nonpaying customers.

The company was founded in 2003. Ebay Inc bought it in 2005 for $3.1 billion.

In 2009, eBay sold a majority stake in Skype to an investor group that included Silver Lake, the Canada Pension Plan Investment Board and Andreessen Horowitz for $1.9 billion in cash and a $125 million note. EBay retained about a third of the company.

HANDICAPPING THE IPO

Last August, Skype filed a registration statement to go public. The October appointment of a new chief executive, Tony Bates, a former senior vice president of Cisco Systems, put the eagerly anticipated IPO on hold until the second half of 2011.

But rivals including Apple Inc and Google have marched into Skype's territory, undercutting the value of the pioneer service.

Now, Skype might again change hands.

Although Facebook and Skype would benefit from each other's large community of users, neither has proven revenue models, said a separate source familiar with the companies.

For Skype, the clock is ticking, as large social media and software companies pour into the public markets.

On Wednesday, shares of Renren Inc, China's largest social networking company, surged nearly 57 percent in its first day of trade.

LinkedIn said on Wednesday it would list its shares on the New York Stock Exchange. The social networking site for professionals filed to raise up to $175 million in an IPO expected later this year.

The flood of Internet public offerings this year will give Skype backers a clearer sense of its prospects, another source said.

"When a company is not going public and it has been on file for a long time, one way or another something is going to happen," that source said.