ITV has sold Friends Reunited for £25m, despite having agreed to pay a total of £175m for it in 2005.
The buyer is Brightsolid Limited, which is owned by DC Thomson, Dundee-based publisher of comics such as the Beano.
The announcement came with the release of half-year results, which were hit by the worst decline in UK television advertising on record.
ITV made a pre-tax loss of £105m in the period. There was no update on who is to take over from boss Michael Grade.
Mr Grade is due to stand down as chief executive by the end of 2009, following a regulatory review.
The £105m loss compares with a £1.5bn loss in the same period of 2008, although last year's figure was hit by a £1.6bn charge, reducing the value of investments made in 2000 and 2004.
ITV's advertising revenues fell by 15%, which was slightly better than the 17% fall in the overall market.
Its results came soon after annual losses were reported by News Corp - which owns the Times and Sun newspapers in the UK and the New York Post and Wall Street Journal in the US.
It said it was to start charging online customers for news content across all its websites, having made a loss of $3.4bn (£2bn) in the year to the end of June.
This included write-downs at its most high-profile online venture, MySpace - which, like ITV, has suffered from lower than expected advertising revenues.
ITV's chief operating officer, John Cresswell, told the BBC that advertising revenues were improving.
"We're down 15% in the first half and in the third quarter, the decline is still pretty tough at -12%," he said.
"But September for us is at -7% so the direction of travel is good."
Mr Cresswell, who is thought to be among those in the running to be the new chief executive, added that "we are taking advertising share from our competitors".
The broadcaster's pension fund deficit had ballooned to £538m by 30 June, compared with £178m at the end of 2008.
ITV is in the middle of a cost-cutting programme, which aims to deliver savings of £155m this year and £285m a year by 2011.
ITV paid an initial £120m for the Friends Reunited website in 2005. It paid an additional £55m earlier this year - a sum which had been dependant on its financial performance. ITV said that the site was profitable.
Friends Reunited was launched in July 2000 from the spare bedroom of Steve and Julie Pankhurst's home in North London.
Its model of helping people to find old friends from their old schools, colleges or clubs attracted millions of users to the site.
But more recently, it has been overtaken by social networking sites such as Facebook, MySpace and Twitter.
Mr Cresswell said that the site had been "bought by previous management with a slightly different strategy".
The company buying it, Brightsolid, already owns findmypast.com, which operates the official 1901 and 1911 census websites.
The purchase of the Friends Reunited Group gives Brightsolid ownership of the Genes Reunited site, which was set up in 2003 and has nine million members worldwide.
Brightsolid said the combination would create Britain's leading genealogy business.
And social media analyst Rax Lakhani said that while ITV had hoped to develop Friends Reunited as a mass-media platform, DC Thomson had "a clear opportunity to reposition the brand as a specialist genealogy platform".
I'm a Celebrity
On the programming front, ITV said there had been successes over the first half of the year, with the final of Britain's Got Talent being watched by almost 20 million viewers, the most for a non-sports programme in six years.
The broadcaster's other top-performing shows included the drama Whitechapel and Harry Hill's TV Burp.
ITV also made more money from selling its content and formats overseas, including a German version of I'm a Celebrity called Ich bin ein Star.
Demand for its online video also grew, with more video views in the first six months of 2009 than in the previous 18 months.
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