When it comes to online advertising, don't forget about Time Warner (NYSE:TWX). Sure, its AOL arm isn't out making the same kind of waves that Google (NASDAQ:GOOG) or Yahoo! (NASDAQ:YHOO) are kicking up lately. Then again, it doesn't have to.

AOL announced this morning that it's buying Third Screen Media. The mobile-advertising specialist's sponsored network generates 225 million monthly ad impressions.

The company's push into wireless-handset ads should be fairly obvious. Yahoo!, Google, and Microsoft (NASDAQ:MSFT) are all making inroads in the nascent market. Microsoft announced the acquisition of European mobile advertiser ScreenTonic just last week.

Mobile ads may never be as lucrative as the major portals' paid-search strongholds, but they do give companies something more to sell. AOL will now be able to offer a suite of graphical ads, search-result text ads, and mobile ads to advertisers.

AOL hasn't been asleep at the wheel. Over the past year, it has acquired web-chat software maker Userplane, online content aggregator Relegence, and broadband-ad enabler Lightningcast. Even though Google owns a 5% stake in AOL, Time Warner's online subsidiary is still set on being a freethinker.

Moves like today's Third Screen Media acquisition, or last month's initiative to place graphical ads on e-mail-sent confirmation windows, should keep the company and its Advertising.com hub growing. If you decide not to notice, well, it's your loss.

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Longtime Fool contributor Rick Munarriz has been an AOL subscriber for 15 years. He does not own shares in any of the companies in this story. He is also part of the Rule Breakers newsletter research team, seeking out tomorrow's ultimate growth stocks a day early. The Fool has a disclosure policy.