My experience is, not very much.
While working with MM3 we used Second Life (that was then marketed as the next big thing) for pushing news to a screen within the official Swedish area that visitors could read. A very nice feature that we didn’t fully utilize, but it was neither any point commercially speaking.
When I revisited Second Life today it felt very lonely. Even the supposed most popular spots were quite empty. Also, the technology hasn’t evolved: Environments still look quite simplistic, despite years of evolution of 3D technology.
Maybe we’ll see something new in this direction, like Facebook or Twitter in almost real life, but I don’t think they have any such plans. It doesn’t fit with the now established one-to-arbitrarily-many blogging paradigm.
Maybe EA could collaborate with Linden Research for a multi-user Sims. It would probably spell chaos.