Investors are starting to treat Shopify Inc. with a bit more skepticism than they did in the past.

The Canadian e-commerce company’s stock sank more than 6 per cent in early trading Tuesday despite beating estimates for first-quarter revenue, profit and full-year revenue. Gross merchant volume, the total amount of goods sold on all Shopify-run sites, grew 64 per cent from a year earlier, compared with an 81 per cent increase in the same quarter last year. That metric has become a key number investors zero in on.

Shopify beat estimates for revenue, making US$214.3 million in the three months ended March 31, versus the average analyst estimate of US$202.4 million. The Ottawa-based company reported a profit of 4 cents a share, far outstripping the average projection for a loss of 5 cents.

To Shopify bulls, the company’s model of adding users, making it easier to sell online and then selling them added services as they grow, reminds them of Amazon’s "flywheel" approach, where revenue snowballs as more people use the platform. But to doubters, Shopify’s growth is too tied to small sellers who quickly leave the site only to be replaced by others. Once the company churns through enough people, there won’t be many more left to keep up growth, according to the argument advanced by shortseller Andrew Left.

Left’s Citron Research released a report in 2017 that called Shopify a “get-rich-quick” scheme. The stock fell 12 per cent, the most ever, and shook the narrative that the company could do no wrong. But shares rebounded after Chief Executive Officer Tobi Lutke called Left a “troll” and dismissed his accusations. Shares were up 32 per cent so far this year at the close on Monday.