Meebo, the leading social network and IM chat platform, has announced that it will begin to sell advertising across 40 top publisher sites like AddictingGames, IGN.com, and Current TV.

It was only a matter of time before Meebo launched this new ad network. They had the popular brand recognition — and now they have the audience reach. For their large publisher partners, they offer a chat services that enables users to directly connect and chat on sites. The ad units are to be displayed right within this enterprise chat service. So while the Meebo.com site itself only boasts 4.1 million unique users, which is relatively small in online publishing, Meebo with the addition of 40 sites has a very respectable reach of over 45 million unique users.

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What I like most about this announcement is Meebo’s ad unit of choice: a rich media/expandable 900×400 banner.  Though they would have liked acquired more ad sales fill rates with standard IAB ad units (like the always in fashion, 300×250 ad unit), Meebo instead is taking a smart risk — and letting advertisers share in the reward. For brand advertisers, Meebo offers a lot of interactivity within its ad units via a display ad beside a video promotion, clickable thumbs, and a send to friend feature.

Sure, the larger the ad, the higher the probability of annoying users. But Meebo is offer such a valuable service for free that I think the consumer backlash will be minimal. Also, the ad is expandable only when the user initiates it by hovering over it. Smart!

I wrote previously that at a time when brand advertisers are cutting back spending, it’s more important than ever for ad networks to differentiate. If you think about it, any ad network can offer you audience reach, standard ad units, and strong publisher sites. But only a select handful of ad networks, now Meebo included, can offer advertisers a chance to get creative and interact at a high level with their audience. Meebo ads are creative and presented in the right context: It’s an interactive ad presented to users when they are in the act of interaction (with friends and peers). Love it!

I’ll likely be reaching out to them over the next few weeks for a Q4 test ad buy.