After setting up buzz, if you don't change the default settings, others can see who you most frequently (not sure about the most frequent part, I guess they pick almost all the contacts that you ever had conversation with if your contact list is not too long) chat with or email to due to the default automatic friends feature. Looks like they have not learned from the Facebook beacon experience -- when it comes to information sharing it is safer to opt-in rather than opt-out.
The above link mentions that:
"Imagine ... a wife discovering that her husband emails and chats with an old girlfriend,"
(Btw, if you are honest, you probably don't need to hide anything. Are we encouraging people to be dishonest by allowing them to hide behind the screen in the name of privacy??)
Also mentions that:
"Imagine ... a boss discovers a subordinate emails with executives at a competitor."
(When you use a free service like Google mail/chat, you don't have much control over your information - your profile, your chat logs, your contacts, your emails ... this raises the question if we should use such services for business purposes or highly private matters??)
There could be other damaging inferences as well. For example, if Bob frequently communicate with one of his doctors, John, who specializes in cancer treatment. Others will be able to infer that Bob is possibly having some sort of cancer.
There are some mitigating factors, however. Buzz only shares information about other people who are using Buzz and have set up public profiles in Google. So currently, most Gmail users are not publicly listed by the service. Users can also "unfollow" people who they don't want to be linked to.
You can follow the steps in this to change the default settings.