In a posting on Twitter's advertising blog, Product Manager Kevin Weil unveiled the new option, which allows brands to reach specific, but not necessarily all, followers.

Reach N.Y., Not Nebraska

"What if you want to make an offer just to New York Twitter users?" he asked. Weil said that, previously, it's been "impractical to send these kinds of highly tailored Tweets, since there was no way to reach people in New York without also reaching followers in Norway, Nebraska and Nigeria who can't take advantage of your offer."

The new advertising allows advertisers to direct messages to users by location, device, or platform. Twitter said that an example use might be when a global brand had different launch dates in different countries, with messages tailored to each country. Mobile apps providers might also want to reach only smartphone and tablet owners, not those on desktop computers.

The company said that it has previously been testing the new offering for several weeks with British Airways, Coca Cola, The Washington Post and Wendy's. It is now available to all advertisers who use Promoted Tweets.

As with other Promoted Tweets, advertisers only pay when users engage with the ads, and the ads with the most engagement are shown most often. Engagement occurs when someone retweets, replies to, clicks or favorites a Promoted Tweet, while impressions -- without engagement -- have no charge. Promoted Tweets are also designed to reach users when they're searching on Twitter, or they can be targeted to users' timelines.

'Jury Is Still Out'

We asked Andrew Frank, research director for media, marketing and advertising at Gartner , if the new targeted Twitter ads were likely to increase the appeal of businesses advertising in that medium.

He replied that it's not yet clear, although, in general, "the more targeting you can have, the more valuable it can be" to advertisers. Frank said that, at any rate, it's likely to "be a benefit to Twitter, because they can be more efficient about their inventory."

Frank also noted that "the jury is still out on how effective Promoted Tweets are." There are some areas where it could potentially be effective, he said, but "there are still questions about whether they should be part of a mainstream advertising portfolio."

Twitter has been actively increasing its value to advertisers and outside developers. Last month, for instance, it began blocking the display of tweets on LinkedIn, saying that it intended to "more thoroughly enforce" its Developer Rules of the Road with partners. Twitter indicated that it wanted more control over its platform and "the core Twitter consumption experience."

In June, the company launched event-oriented hashtags in partnership with NASCAR, and it ran its first-ever TV commercial. The commercial concluded with Twitter.com/#NASCAR, and the race being promoted was officially called the Pocono 400 Presented by #NASCAR.