There's often a lot of hype surrounding Robinhood stocks, and sometimes that can help carry share prices to astronomical heights. But that isn't happening of late with two of the platform's more popular holdings, Facebook (META -0.82%) and Tilray (TLRY).

Since the start of September, both stocks are down by about 15% and doing much worse than the S&P 500, which has declined by just 7% during the same period. But are these top Robinhood stocks in trouble, or is now the time to scoop up these deals before their share prices rally? Let's take a closer look at the businesses and find out.

1. Tilray

Tilray's stock is down 26% in September, and yet the company hasn't released any news since it reported its second-quarter results on Aug. 10. One of the problems with investing in pot stocks is that industrywide problems or concerns can often send stocks reeling. The Horizons Marijuana Life Sciences ETF (HMLSF 0.28%) is down 13% this month, a more modest decline than Tilray's. What's surprising is that the British Columbia-based company is coming off a decent quarter during which it reported sales of $50.4 million for the period ending June 30. That's up 10% from the previous year. And while it may not be a huge increase, some cannabis companies struggle to generate any top-line growth whatsoever.

Financial chart with falling arrow

Image source: Getty Images.

The company's stock isn't performing poorly due to any bad news or earnings report. One theory is that its share price was just expensive, and with cannabis stocks struggling as a whole in September, Tilray suffered a more significant decline due to its valuation. However, that doesn't pass the sniff test, as Tilray's stock is trading at a comparable price-to-sales (P/S) ratio to its Canadian-based peers (Canopy Growth is the big exception here):

TLRY PS Ratio Chart

TLRY PS Ratio data by YCharts

It's difficult to gauge why Tilray's stock has tumbled so much in September and possibly fallen out of favor with Robinhood investors, especially given that the decline in price has been fairly steady.

Part of it could be related to a more negative outlook on the hemp industry. In August, the U.S. Drug Enforcement Agency (DEA) released proposed rules for hemp that could put companies at risk. A key problem is that during the extraction process, hemp-based cannabidiol products may temporarily exceed 0.3% of tetrahydrocannabinol (THC); above that percentage, regulators classify the substance as marijuana, which is illegal. THC is the psychoactive substance in cannabis, and hemp products focus more on cannabidiol (CBD), which is more closely associated with the plant's health benefits. Under the DEA's proposed rules, companies could potentially run into trouble if THC levels of products spike above 0.3% even temporarily. The rules aren't final, but they've been enough to spook investors of late. And with hemp accounting for more than 40% of Tilray's sales thus far in 2020, it's a big part of the company's business.

Between the concerns relating to hemp, modest sales growth, and Tilray incurring a loss in each of its past 10 quarterly results, there is no overwhelmingly strong reason to buy the stock today. Simply declining in value doesn't make Tilray a stock worth buying.

2. Facebook

Shares of Facebook are down just under 15% in September. That's a more modest decline than Tilray's, but it's enough to send the social media stock down to where it was back in early August. Tech stocks in general are struggling this month as investors are growing more concerned with high valuations. The NASDAQ itself has lost 10% of its value in September.

Using the P/S ratio again and comparing the stock against its peers, it's clear that shares of Facebook aren't cheap:

FB PS Ratio Chart

FB PS Ratio data by YCharts

With the country in the midst of a recession and COVID-19 creating lots of uncertainty, even high-risk Robinhood investors may no longer be willing to pay large premiums for these types of stocks.

On July 30, the tech giant released its second-quarter results for the period ending June 30. Its revenue remained strong at $18.7 billion, up 11% year over year. The result was encouraging amid a pandemic, but it wasn't anywhere near the 27% sales growth that the company generated for 2019, when sales topped $70.7 billion.

What's especially surprising is that investors continued to buy shares of the company, even though more than 1,000 advertisers joined a boycott of Facebook because they believe it doesn't go far enough in keeping hate speech and misinformation off its platform. With many companies starting to cut back spending on Facebook ads in July, however, investors may not see the effects of the boycott until the company releases its next quarterly results, which will likely be next month.

Should you invest in either of these stocks?

Here's a look at how Tilray and Facebook are doing in 2020:

FB Chart

FB data by YCharts

These two stocks are going in opposite directions, with Facebook outperforming the S&P 500 by a wide margin while Tilray's doing even worse than the Marijuana Life Sciences ETF. However, neither stock is worth picking up on the dip. While Robinhood investors love growth stocks, these companies just aren't generating enough revenue of late to warrant more buying activity.

Facebook has risen in value even as advertisers grow frustrated with the social media company. And in a year when people are cutting back spending amid the coronavirus pandemic, the last thing you want to do is alienate customers who are still spending money. According to research company Zenith, global ad spending will fall by 9.1% in 2020.

Tilray, meanwhile, is facing no shortage of its own problems. The latest hemp issues only make things even worse, and make the stock an even less appealing investment.

There are some good buys out there with the markets falling in recent weeks, but these are two stocks that Robinhood and any value-oriented investors should steer clear of right now.