Bringing more immediacy to Facebook apps from BuddyMedia, (fluff)Friends, RockYou, Slide, The Broth and the WaterCooler, Web-based instant messaging startup Meebo is now letting Facebook developers add chat to their apps through a new partner program. Basically, these are versions of Meebo Rooms inside Facebook that can be customized and skinned any way the app developer wants. They integrate with Facebook’s registration system (no Meebo account required), include traffic and advertising tracking, and automatically create new overspill chat rooms when one fills to capacity.

Meebo also announced that it is reaching more than 20 million people a month, most of those through Meebo widgets like the ones that will be created through this partnership with Facebook app developers. Meebo.com itself gets 6.5 million unique visitors a month, according to the company. The rest is widget traffic.

Here is what eBuddy’s Acebucks Sudoku game looks like in Facebook with chat underneath.