Investors in Facebook, still reeling from the social network's disastrous stock market debut, are braced for further losses as the ban on early backers selling their shares begins to lift this week.

In a staggered process which begins on Thursday and peaks in November, about 1.9bn shares – four times the current publicly traded number – will begin to be released from "lockup".

Facebook's high-profile owners, from Microsoft and Goldman Sachs to U2 frontman Bono's venture capital fund Elevation Partners, will be free to sell the billions' worth of securities they held back from the initial public offering in May.

"It remains a real risk that shares will be sold on to the market and temporarily depress prices as the market could struggle to absorb an expanded float," said Brian Wieser of Wall Street research group Pivotal.

Among the first out of the gate will be Accel Partners, the Silicon Valley venture capital fund which bought in when Facebook was worth just $98m (£62m) and whose holding is worth $3.3bn at today's prices. Another large investor able to reduce his position from this week is Yuri Milner, the Russian tycoon behind two companies with large holdings. Milner bought in when Facebook was worth $10bn, and again when the valuation had risen to $50bn.

Estimates suggest that half of the 268m shares eligible for sale from Thursday could be auctioned. The impact may not be felt immediately. Accel and Milner, who are believed to control the majority of these, are likely to make any disposals in a controlled way, either by staggering the sales over several days or by placing blocks at a pre-agreed price with institutions.

The major threat to Facebook shares, whose value has fallen 40% in the three months since their debut at $38, comes in November, when 1.2bn shares, many of them belonging to employees and venture capital firms, are released from their trading moratorium. Pivotal believes that these types of investor could decide to sell up to three-quarters of their holdings.

"This date becomes the greatest wildcard," said Wieser. He believes a lot depends on Facebook's next set of financial results, expected in October, which should show an acceleration in its rate of growth that is likely to continue until Christmas and beyond as the company expands its sources of advertising revenue. "They may be able to navigate through this. The more they can convince Wall Street of the merits of their story, the more they are likely to generate demand for the shares."

Those thought likely to hold on to their investment are Facebook evangelists who already have large personal fortunes. Included in this category are Mark Pincus, founder of games group Zynga, Sean Parker, the Napster founder and former Facebook president, and LinkedIn founder Reid Hoffman.

Others paid a high price before the IPO and may decide to sit tight. Goldman Sachs, which organised a pre-float investment round on behalf of private investors, many of them from Europe, bought in when Facebook was thought to be worth $50bn, well above today's $46bn market capitalisation.

Richard Hunter, head of equities at Hargreaves Lansdown, said: "If you get a lot more sellers, that is going to potentially push the price down. But at these share price levels it may well be that it is a trickle rather than a flood into the market, which wouldn't depress the share price any further."

He said today's price may already be factoring in the "overhang" in the shares and draws a parallel with Lloyds and Royal Bank of Scotland, in which the British government has a large stake: would-be investors are deterred by the fact that a swath of the stock could suddenly flood the market.

The overhang will not be eliminated until next year, when the final lockdown ends and the last 47m shares held by Milner's vehicles, including London-listed Mail.ru, are released from their restrictions.

Groupon punished

Shares in daily deal site Groupon plummeted in after-hours trading on Monday as the company announced second-quarter revenues below Wall Street expectations. The Chicago-based firm reported revenues of $568.3m for the quarter, compared with $392.6m for the same period in 2011. Analysts had expected revenues of $573m, according to Thomson Reuters. Groupon shares fell more than 15% to $6.39. The stock is down about 60% this year. The company reported a profit of $28.4m, outweighed by investor fears that the appetite for online deals is slowing.