The value of Linden Lab, which operates the virtual world Second Life, has plunged by more than 21%, according to SharesPost, which tracks the private secondary market.

In late June, Linden Lab brought back founder Philip Rosedale as interim CEO, after CEO Mark Kingdon stepped down.

The current value of Linden Lab is estimated by SharesPost to be about $271 million or about $100 million less than a year ago.

The plunge in value appears related to today’s news that the company is closing the five year old "Teen Second Life" virtual world at the end of this year.

The company said:

"…supporting and developing for two separate grids has been a challenge for us, and has slowed progress on improvements that benefit all Residents. To help us focus our resources and development on the Main Grid, we have made the difficult decision to close Teen Second Life."

Linden Lab has lowered the age for Second Life membership to 16 years and is evaluating lowering it further to 13 years — but only if it can develop ways to provide safe access for younger teens.

"Linden Lab has received over $19 million from Mitch Kapor, Catamount Ventures, Benchmark Capital, Ray Ozzie, Omidyar Network, Globespan Capital Partners, and Bezos Expeditions."

SharesPost estimates the value of private firms based on shares bought and sold in private markets.

– Facebook has a value of about $25.5 billion;
– Zynga is valued at $4.9 billion;
– Twitter at $2.2 billion;
– LinkedIn at about $2 billion.

[Editor’s Note: Tom cross-posted this to Silicon Valley Watcher. –mrh]