Google Buzz since it launched. I'm absolutely seeing it as a replacement for FriendFeed. Buzz appears to be close enough that I have shut down my FriendFeed account and don't intend to look back. I've pretty much stopped using Twitter as well. I didn't stop using Twitter intentionally, I just find it much less interesting today than I did last week.
That said, it appears to me that Google has done something brilliant by tying Buzz to your Gmail account. They have made a statement about online persona-- that people participate in social media as individuals. Shared email accounts are a security nightmare (among other things). They aren't going to be allowed to happen in organizations without a great deal of consternation and forethought. A Gmail account gives you access to a lot more than just email. No Gmail account-- no participation in Buzz.
Google has done a very googly thing with Buzz. There is an algorithm that decides what you see and what others see from you. No one really knows how it works. We do know that it learns, and that it requires active participation. I'm thinking any organization that might try to use Buzz for broadcast, rather than conversation, is destined for obscurity. If a buzz falls in the forest... We all will have to consider our BEO (Buzz Engine Optimization). Buzz will require organizations to come out from behind the curtain and talk as individuals. If not, all those official social media channels will be lost in the flow and buried deeper than a blog post with no links.
The thought of this kind of makes me smile. Thank you Google for putting the social back in social media.