Nic over at Kzero has put together a pretty neat slide show of the major brands in Second Life. Remember though, I think everything happening right now in this world needs to be treated as experimental. If you're looking for profit, you're 5 to 7 years early.
This space has no analogue, especially to the current 2D web, so stats that work there do not apply. Building the 3D web will be fun, and these brands are helping it along. They get to have input.
Check it out: The definitive guide to brands in Second Life