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Twitter has already been showing some limited advertising on its mobile apps, but will now start showing its "Promoted Tweets" -- its primary ad product and its attempt to replicate Google's AdWords program -- on phones as well.
Just like the Promoted Tweets that Twitter shows on its primary Twitter.com site, these ads will show up in users' "timelines."
At first Twitter will only allow advertisers to place the ads in front of users who are already following their accounts. But within months it will expand the program to allow marketers to reach people who don't follow them, just as it does on the web.
The announcement comes a day before a Facebook marketing event where that social network is widely expected to roll out a mobile ad product of its own, so the timing may be more than coincidental.
But the bigger picture is that Twitter needed to increase its ad inventory anyway to accommodate a wave of new ad buyers from its self-serve ad program.