Microsoft Chief Executive Officer (CEO) Steve Ballmer (L) and Skype CEO Tony Bates shake hands at their joint news conference in San Francisco, May 10, 2011. [Photo/Agencies]

NEW YORK - Skype's owners, led by private equity firm Silver Lake , are set to earn more than three times their investment, for a total capital gain of more than $5 billion, on the sale of Skype to Microsoft Corp, a source familiar with the situation said.

The gain is particularly large considering the short amount of time the investors have owned internet phone service Skype -- around 18 months -- since buying a majority stake from eBay. That is a short turnaround for private equity which typically invests with a time horizon of three to five years.

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The $8.5 billion price Skype is commanding from Microsoft reflects the premium being put on hot social media properties and the number of potential buyers.

Microsoft's deal to acquire Skype was announced on Monday.  

Skype, which was considering an IPO, sparked interest from parties including Facebook and Google according to sources.

Cisco also was interested, a source said. Cisco was not immediately available for comment.

One concern Skype and its owners may have had was whether they were missing out on getting substantially more, giving escalating valuations put on companies such as Facebook and Groupon.

That could explain the six weeks it took for Microsoft and Skype to reach a deal. Microsoft decided to make an unsolicited offer with the final price by April 18, it said.

"It's not like the deal came together in five days, it came together over the course of the last few weeks," said one source familiar with the deal. "They had time to digest the price, they had time to understand the parameters of the contract and then ultimately figure out the final pieces."

For Microsoft, the stakes were high. The world's largest software maker has been beset by a string of setbacks including the failed acquisition of Yahoo Inc and a halting start in the mobile phone arena now dominated by Google Inc and Apple Inc.

It decided against hiring investment bankers to help them with the deal. Microsoft said it decided to appeal directly to Skype's ownership base with Silver Lake taking the lead.