The 57% drop came after the Kyiv Post newspaper of Ukraine, where Cupid has large back-office operations, published a claim that the Edinburgh-headquartered firm employs "motivation managers" to entice visitors to its websites to spend more on subscriptions.
Cupid has been a darling of the stock market since it floated at 60p in the summer of 2010, when it was called Easydate, and its shares were trading at 250p just six months ago.
But the Alternative Investment Market-listed stock has come under pressure since allegations concerning its tactics emerged in January.
Yesterday, after the latest claims, the stock changed hands for as little as 38p before finishing down 65p at a record closing low of 49p. This cut its stock market worth from £95m to £40.9m in just one trading day.
Mr Dobbie, who has consistently stated that Cupid behaves in a "legal and appropriate manner", had sought to boost confidence in the company's prospects by buying another £1m of shares earlier this month. But yesterday he saw his holding of 14.7 million shares fall in value from £16.8m to £7.2m.
Max Polyakov, the Ukranian who co-founded the company with Mr Dobbie in 2005 and remains its largest investor despite having left last year, was the biggest loser from Cupid's share slump. His stake fell by £12.1m to £9.1m.
Cupid, which operates sites such as Be Naughty and Girls Date for Free, faced allegations aired by the BBC earlier this year that customers were lured into subscribing to its sites after receiving messages from users who later disappeared.
In a stock market announcement yesterday Cupid, responding to the Kyiv Post article, pledged to investigate a claim that those who signed up to a three-day trial membership were encouraged through messages created by Cupid employees to move to a full subscription.
It said this would be addressed through an independent audit by a large accountancy firm that it had previously announced.
Cupid confirmed to investors yesterday that it employed a "motivation team" of 24 people. But it insisted that it does not communicate with members who have not paid to take out a subscription.
But the company said it does contact new paying subscribers "to help them get the most out of the site". It also communicates with subscribers "in order to detect and identify any technical or product issues pro-actively," Cupid said. "The team also moderates chat rooms and forums," it added.
Mr Dobbie, who also co-founded and floated Scottish data storage firm Iomart, has previously blamed short sellers for targeting the company.
The latest filings made public by the Financial Services Authority show that it remains in the sights of the hedge funds, with Tremblant Capital of New York disclosing a 3.6% short position and London-based Ennismore Fund Management a 1.3% bet on Cupid's share price falling.