What: Shares of e-commerce company Wayfair (NYSE:W) tumbled on Monday on no specific news. While the volatility of the stock market was certainly a contributing factor, Citron Research's tweet about the company having "the worst business model on the Internet" on Feb.5 may still be weighing on the stock. At 11:30 a.m. ET Monday, Wayfair stock was down about 9.5%.

So what: Back in August, Citron Research published a report calling Wayfair the most mispriced stock it had seen in years. Citron compared Wayfair to Overstock.com, which has a similar business model, pointing out a big discrepancy between the valuations of the two companies.

On Friday, Citron tweeted that it believes that Wayfair is worth $5 per share, about $30 lower than the current share price:

Wayfair is growing its revenue very quickly. During the third quarter, the company posted 76.7% year-over-year revenue growth, bringing its nine-month total to over $1.5 billion. However, the company is unprofitable, posting a net loss of $62 million through the first nine months of 2015. Wayfair's market capitalization is roughly $3 billion.

Now what: It's unclear whether Citron's tweet is the main reason behind Wayfair's decline on Monday, given that its opinion has been known since August and that various high-flying Internet stocks have taken a beating so far this year. Wayfair's profitability has been improving, but Citron is concerned that a combination of intense competition and high levels of marketing expense will prevent the company from ever producing a meaningful profit.

Of course, the opinion of a single analyst isn't a reason to buy or sell a stock, and investors should always do their own research before making any investing decisions.