Stats from internet analytics company ComScore suggest that the Facebook-owned service had 7.3 million daily visitors in the US during August, while the microblogging site hosted 6.9 million.
Instagram users also spend more time using the service, with the average mobile visitor spending 257 minutes logged in, compared to Twitter's average of 170 minutes.
This is the first time Instagram has seen more mobile traffic than Twitter, which has a significantly larger user base of 500 million. The 2-year-old photo service currently has 80 million worldwide subscribers.
Moreover, Twitter's number of monthly unique users was higher at 29 million, suggesting that Instagram account holders are more likely to return to the site repeatedly and spend more time there. Around 22 million individuals used the service in August.
Facebook officially closed its acquisition of Instagram earlier this month after receiving the green light from regulatory bodies in the US and Europe.