Instagram's growth has accelerated markedly in the last year.
Last month, the number of people who use Instagram Stories every day reached 250 million, up from 200 million in April. That's a growth rate of 25 percent in just one quarter.
It's also well ahead of Snap, which reported 166 million average daily users in the first quarter of 2017, for a year-over-year growth rate of 36 percent.
Apart from Stories, the overall Instagram service also has been growing fast. In April, Instagram said it had 700 million users, up from 500 million in June 2016. That's an addition of 200 million in 10 months.
For most of 2014 and all of 2015, Instagram had been adding roughly 100 million every nine months.
"There was a major push within Facebook to get more users on Instagram," said Patrick Moorhead of Moor Insights & Strategy, an industry analyst firm.
Facebook did so by sending messages to users telling them which of their friends were already on Instagram.
"It was the right strategy and it worked," Moorhead told CNBC.
Now even Morgan Stanley, which was one of the investment banks that helped underwrite Snap's IPO, is admitting in a note this week that "Instagram is likely to be more disruptive than previously expected" to Snapchat's prospects.
The stock downgrade and revenue-estimate cut was a rare move on Wall Street given that the giant investment bank helped sell Snap's IPO just four months ago.
And Facebook will keep turning up the heat on its smaller rival.
Some Instagram advertisers are now getting for free a type of sponsorship that they have to pay for on Snapchat, according to the Morgan Stanley note.
"It's going to be hard for Snap to become more than a niche player" in the online ad market, Pierpont said.
Disclosure: CNBC parent company NBCUniversal is an investor in Snap.