Robinhood, the popular stock-trading app that reportedly landed a $5.6 billion valuation recently, claims to have 4 million registered users.

The company recently rolled out a web platform featuring a suite of new features originally unavailable on its sleek smartphone app.

But data showcased on the web platform also raises questions about the strength of its user base. Apple is the most popular stock, with about 141,000 owning it on the platform, according to the data. That represents just 3.5% of the platform's 4 million registered users.

Five of the top 10 stocks on the platform have share prices below $15, which could suggest that many investors are strategically buying the lowest-priced stocks.

In addition, two of the most popular stocks in the top 10 trade below $10. That could be because Robinhood has a referral program in which users can get a free stock when they invite friends to the platform, according to one person familiar with the matter. There's apparently a 98% chance that the free stock will be worth less than $10.

"Our mission is to democratize access to America's financial system, and we're thrilled that all types of investors choose to trade on Robinhood," the company said.

Robinhood, the brokerage startup known for pioneering free stock trading, reportedly landed a big $5.6 billion valuation recently.

It's adding new functions, such as crypto and options trading, at a breakneck clip. And its reported user growth is something its competitors on both Wall Street and Silicon Valley envy. At last check, the company claimed more than 4 million users with fully registered accounts. It launched in 2013.

The company's newly rolled out web platform, which features a suite of features originally unavailable on its sleek smartphone app, is the latest service from the company.

But data showcased on the web platform also raises questions about the strength of its user base.

According to a list of the most owned stocks on the platform, about 140,800 Robinhood users own Apple — the most valuable company in the world and the most popular stock on the platform.

The next most popular stock, Ford, was owned by 127,000 users. GE rounded out the top three, with 122,500.

Based on those numbers, only 3.5% of Robinhood's 4 million registered accounts hold Apple.

"That's pretty surprising to me," Mike Dudas, a serial entrepreneur who cofounded the tech company Button, told Business Insider. That view was echoed by several industry insiders.

"It would shock me if less than 5% of Robinhood's registered users held Apple stock and even fewer held Snap," Dudas added. Snap has 68,000 holders on the platform, according to the website.

To be sure, Apple is a high-priced stock for those looking to make their first investment, closing Tuesday at $175.23. But more-affordable stocks like Ford ($10.99 a share) still have a comparatively small percentage of holders on the platform. The top exchange-traded fund on the platform, the Vanguard S&P 500 ETF, has 23,200 holders.

Five of the top 10 stocks on the platform have share prices below $15, which could suggest that many investors are strategically buying the lowest-priced stocks.

Two of the top 10 stocks, GoPro and Fitbit, trade for less than $10. A person familiar with the matter said their popularity could be due to Robinhood's referral program. Under that program, users who invite friends to the platform can be rewarded with free stocks. For the most part, the stocks given out are cheap, with a 98% chance the stock will be valued between $2.50 and $10, according to the company.

To be sure, just because a stock has a low price doesn't mean it isn't a good investment pick. In fact, the company prides itself on being a home to young, less wealthy investors.

"Our mission is to democratize access to America's financial system and we're thrilled that all types of investors choose to trade on Robinhood," the company said in a statement.

The top 10 stocks on the platform have a combined 1.1 million holders, though many of the investors holding Apple could also be invested in GoPro or Facebook or Twitter.

Brian Barnes, the CEO of M1 Finance, an automated financial adviser, told Business Insider he didn't think it was possible that Robinhood had 4 million funded accounts given the data, drawing a distinction between registered accounts, in which a user has signed up, and a funded account, in which a user has deposited cash to invest.

"Robinhood has 4 million brokerage accounts on the platform," Robinhood said in a statement. "As we expect, not all 4 million people own positions in equities or ETFs at all times."

Listen to Business Insider's recent conversation with Robinhood cofounder Vlad Tenev, here>>