Several companies are already trying to find ways to identify Twitter users who wield the most influence around particular brands and topics. Raj Kadam, cofounder and CEO of Viralheat, a San Jose, CA-based analytics startup that measures real-time conversations on social media sites, says that companies can reach out to these users through direct messages or other personal interactions. Promoted Tweets might serve the same purpose while requiring less expense and effort, he suggests.

But Twitter will also have to figure out how to measure the success of individual Promoted Tweets. Vishal Sankhla, Viralheat's cofounder and CTO, says it could be difficult to figure out who's actually seeing which Promoted Tweets, because the most engaged users access the service in a number of ways: via the Web, via text message, or through third-party applications. Twitter will have to make changes to its application programming interface in order to make Promoted Tweets work properly with third-party applications.

Depending on how Twitter decides to implement Promoted Tweets, computing power could become another challenge, says Michael Rubenstein, president of AppNexus, a real-time advertising company. Tweaking ads based on user behavior and other factors requires a great deal of computation and platform stability, he says. It is unclear how much Twitter will try to adjust the behavior of Promoted Tweets on the fly, but Stone's post suggests that ones that don't perform well will be removed.

Promoted Tweets won't appear in user timelines for a while yet. By the time they start appearing, Twitter hopes to have fine-tuned its algorithms so that users don't mind--or even like--the ads.