Some say a picture is worth a thousand words. How about a thousand tweets?

Looks that way.

A new report released Friday shows that Instagram has taken the lead in mobile engagement, topping Twitter for the first time.

According to ComScore, “Instagram had an average of 7.3 million U.S. smartphone users accessing the application every day” in August compared to “Twitter’s 6.87 million U.S. smartphone users accessing the app every day.”

One reason for this could be because some of the most active users on Twitter access the site on desktop, whereas Instagram users are only on mobile devices.

ComScore also found that Instagram is accessed more on mobile than Twitter.

 ”..the average Instagram user spent 257 minutes accessing Instagram in August, compared to 169.9 minutes spent on Twitter,” according to the report.

Good news for Instagram and Facebook, but what about Twitter?

The live-reporting and link-sharing tool might not be reaching the same engagement levels as its rivals, but it is leading the game in mobile advertising. Earlier this month, Mashable ran a story that predicted Twitter would  hit $129.7 million in U.S. mobile ad revenues this year compared to $72 million for Facebook.

Even though eMarketer predicts Facebook will eventually catch up, Twitter continues to pursue mobile ads.

“Twitter CEO Dick Costolo noted earlier this year that the company has generated more revenue from mobile ads than desktop ads on certain days,” according to Mashable.

It will be interesting to watch all three of these social networks grow and experiment with mobile advertising, but I worry any ad presence on Instagram might diminish the engagement. After all, Instagram is supposed to be “cool.”

“You don’t want to ruin it with ads because ads aren’t cool.” – Sean Parker Character in The Social Network