Victims aged in their 20s have each lost an average of £8,900 after falling for investment scams that appear on image-sharing platform Instagram.
Action Fraud, a UK police-led awareness centre, said there had been a surge in activity in recent months by fraudsters posting about get-rich-quick schemes.
Victims are promised high returns within 24 hours, but the fraudsters demand fees and then disappear.
Some 356 reports of losses have been made in the past five months.
Those tricked lost a collective total of more than £3m, but more is expected to have been stolen as some victims may not have reported their losses.
The scam sees schemes advertised via the Instagram app. Those targeted are encouraged to transfer £600 and are promised almost instantaneous profits. Once the money is paid, they are sent images supposedly of profits building up in their accounts.
The fraudsters tell their victims to “invest” more, and that the money can be released for a fee, which is why losses can build to thousands of pounds.
However, they then they close the Instagram account, stop all contact, and disappear with the money.
Investment fraudsters often use professional-looking images and may promise free research reports, special discounts and “secret” stock tips.
ZeroFox, a security company specialising in social media, previously told the BBC that it found more than two million public Instagram posts that push these types of scam, known as money-flipping.
Inspector Paul Carroll, of Action Fraud, said: “Opportunistic fraudsters are taking advantage of unsuspecting victims who are going about their day-to-day lives on social media.”
He urged social media users never to send money to strangers only encountered online, to check financial matters with family members, to only deal with financial firms authorised by the regulator – the Financial Conduct Authority – and to report any cases of fraud.