This Clickable Culture post takes a look at the Unype application that welds multi-user capability onto Google Earth, including 3D avatars and a Skype-powered contacts-list. From the article: "Developed by Holoscape, the Unype client software (currently in beta) connects Google Earth, Skype, and an Unype Server, where other Unype users are logged. Users select their own COLLADA-format avatar models to use in the system, as well as public profile information such as a URL and photo.
Collaboration between users is facilitated through the synchronizing of Google Earth layers between users, establishing a follow/lead link between users, and the sharing of Google 3D Warehouse models from a private site or directly from Google. I wasn't able to get Unype working on my computer, so I'm not sure if Unype's use of Skype includes voice communication in addition to contacts. Seems like a no-brainer, but the Unype site didn't list voice chat as a feature.
Holoscape also makes an Unype Facebook application which allows users to see, move towards, and contact other Facebookers using a 2D map (Google Maps API) and inside Google Earth."