One recent estimate from eMarketer has even suggested that Twitter will generate more revenues from mobile ads than Facebook this year.
But new data show the scale of the threat that Twitter faces from explosive growth at Instagram, the photo-sharing app acquired by Facebook.
Figures from ComScore (first reported by AllThingsD) show that while Twitter still has more monthly unique users on mobile, Instagram is gaining fast â€“ and has already overtaken it in daily usage.
Twitter had 29m unique visitors in the US on mobile in August, according to ComScore's Mobile Metrix, compared with Instagram's 22m. But on a daily basis, Instagram's numbers have leapt from 886,000 in March to 7.3m in August.
Users spent a total of 5.6m minutes in Instagram's app, surpassing Twitter's daily 6.9m users and 4.9m minutes.
As the chart above shows, Twitter's growth rate has been steady but less spectacular than Instagram's over the summer. At 257 minutes per month, Instagram's average engagement time per user is also much higher than Twitter's 170 minutes.
Usage figures declared by Facebook and Twitter themselves tell a slightly different story but also suggest that Instagram is narrowing the gap with Twitter.
Mark Zuckerberg, who struck the deal with Instagram's Kevin Systrom, said at the recent TechCrunch Disrupt conference in San Francisco that Instagram had more than 100m users. That compares with Twitter's most recent active user number of 140m, although this dates back to March.
Instagram's rapid rise will make little difference to Facebook's mobile revenues in the short term. Although some brands such as Burberry are very active, it carries no ads and because activity is contained within an app, it's not a great source of traffic or direct sales at the moment.
But as the rise of Pinterest and Tumblr has also demonstrated, pictures are becoming the atomic unit of social networking. For many people, photos are an even simpler way of sharing their activity with friends than 140 characters.
Twitter is working to simplify its user experience but it still leaves many people baffled when they first start to use it. The appeal of a full-screen photo, however, is more immediate and Instagram's interface is second to none.
Any time spent on Instagram is time not spent on Twitter, and while the use cases for the two services may differ, the photo app risks sucking away some of Twitter's oxygen.
Both Twitter and Facebook saw Instagram usage exploding on their platforms earlier this year, leading to the small company's acquisition for an eyepopping initial valuation of $1bn (although by the time the deal closed last month, that cash-and-stock price fell to around $750m as Facebook's shares declined).
Twitter had made approaches to Instagram before Facebook swooped. Yet the speed of the Facebook deal caught Twitter by surprise. According to one person familiar with the discussions, Twitter had gone as far as indicating the sort of price that it was prepared to pay for Instagram, but before it received any response, Facebook had already sealed the deal.
ComScore's report is just one datapoint but it suggests that as a defensive manoeuvre at least, acquiring Instagram may prove to be a very smart move by Zuck â€“ and a big miss for Twitter.