Ever wanted to instant-message with all of your friends, across social networks? Starting today, you can chat with your friends on MySpace and Facebook, through online IM aggregator Meebo. The way it works is that you sign in to Meebo’s home site using your identity on either social network. Meebo already integrates with AOL’s AIM, Yahoo Messenger, Google Talk and Microsoft Live Messenger, so now you can chat with all of these friends together with your MySpace and Facebook friends from within Meebo.

Facebook and MySpace both offer their own chat services, where they turn your friends on the sites into IM contacts. While MySpace and Meebo worked together to offer this new feature, Meebo figured out how to integrate with Facebook’s IM protocol by itself.

So maybe Facebook will cut off Meebo from accessing its IM service? Or maybe not. Social.IM, a startup recently acquired by iSkoot, has already integrated Facebook IM together with other IM services in a desktop app — and it’s still functional. Social.IM’s service has you login directly to Facebook, and it doesn’t store your Facebook data. Desktop IM aggregator Adium and Meebo rival eBuddy’s mobile service also includes Facebook chat integration.

Meebo, meanwhile, has been busy building IM services for other social networks, such as movie fan site Flixster, teen social network myYearbook, and female-focused social network Sugar Publishing. That service, called CommunityIM, turns your friends on partner sites into IM contacts that you can chat with on those sites — like what MySpace and Facebook do on their own. Meebo has already taken a step towards mass social network-IM integration: You can add your Flixster IM contacts into its core Meebo.com chat aggregator site. Next year, Meebo plans to let users chat across any IM network on any site.

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