They have a rich platform that is network and context agnostic. If they earn a strong position in the IM world, they will have a plethora of ways to leverage their position: business services (thing online support for consumer facing companies), branded experiences, and yes, advertising.
I think they're well on their way to have a firm position in the space. They've got a great product and Time Warner is investing, which is tellingly great for Meebo / tellingly ominous for AOL.
I think they make a good amount of money from all the specialized ads they have. Like the meebo skins, and all those little icons for various bands and products.
