Only 16 of the 99 U.S.-listed companies that were worth at least $10 billion when they went public declined on their first day, according to Dealogic, whose data goes back to 1995.

"A year ago we were trading the stock at 15 bucks a share. And our most recent trades in early June were $55 a share (in the private market)," said Glen Anderson, president of Rainmaker Securities, a secondary trading platform for private pre-IPO shares. Anderson said they traded about $200 million in Robinhood stocks in the private market last year.

Robinhood's easy-to-use interface has made it a hit among young investors trading from home on cryptocurrencies and stocks such as GameStop Corp during the COVID-19 pandemic. Some IPO investors stayed on the sidelines, citing concerns over its frothy valuation, the risk of regulators cracking down on Robinhood's business, and lingering anger with the company's imposition of trading curbs when the meme stock trading frenzy flared in January.

In an unusual move, Robinhood had said it would reserve between 20% and 35% of its shares for its users.

Many IPOs benefit from excluding retail investors, who end up fueling a first-day trading pop by snapping up shares in the open market. By letting many retail investors under the IPO tent, Robinhood made big gains less likely for investors on the first day."The market believes that institutional investors will hold on for a longer time, and retail investors are more likely to flip," said Reena Aggarwal, professor of finance at Georgetown University.

Robinhood shares priced at $38 in the IPO.

Stanford University roommates Vlad Tenev and Baiju Bhatt founded the company in 2013. The two will hold a majority of the voting power, with Bhatt keeping around 39% of the outstanding stock and Tenev about 26.2%.

"It maybe seemed like a good idea to offer (the IPO) to your customers, but it might not be very helpful when it comes to controlling how the shares are allocated and the beginning of the trading of this IPO," said Kathleen Smith at Renaissance Capital in Greenwich, Connecticut.

Robinhood enraged some investors and U.S. lawmakers this year when it restricted trading in some popular stocks following a 10-fold rise in deposit requirements at its clearinghouse. It has been at the center of many regulatory probes.

The company disclosed this week that it has received inquiries from U.S. regulators looking into whether its employees traded shares of GameStop and AMC Entertainment Holdings, Inc before the trading curbs were placed at the end of January.

(Reporting by Echo Wang in New York; Noor Zainab Hussain, Ankur Banerjee and Niket Nishant in Bengaluru; Editing by Arun Koyyur and Howard Goller)

By Echo Wang and Noor Zainab Hussain