Twitter has rolled out its advertising platform called Promoted Tweets. It takes actual tweets from participating brands and pushes them onto the top of relevant search results and shoves them into the stream within third party applications.
Inclusion of this ad system into third party applications is not mandatory but is certainly welcomed by developers that were striving to make a buck on free, ad-supported applications. But, Twitter is missing out on the app up sell.
I use Echofon for both iPhone and Mac. I paid $2.99 for the iPhone application (very much worth it). The Mac application is $20 for the paid, no ads, application and free for the ad-supported application. So far the only ads I've seen inside of Echofon for Mac has been these new Promoted Tweets from Twitter's new ad platform.
Here is where I think Twitter loses. When and if I pay Echofon $20 to remove the ads from the application Twitter doesn't see a lick of it. They also lose ad impressions because Echofon will not display these Promoted Tweets anymore.
I'm not so sure that Twitter needs to care about this because the number of impressions they can count on from their site, search and stream injection is probably much higher than I could guess and growing steadily. But, I still feel they are leaving a bunch of money on the table somehow.