It's already in Gmail.
I presume they'll have a standalone Buzz application at the 2.0 or something similar. That will let an established gmail-only social graph generate lots of activity and use, seeming enticing to yahoo and hotmail (and all third-party email account providers) users that can now join/be invited in on the social-web-2.0-sharing fun.
I mean, I might be exaggerating, but among my friends I can name at max 10 people who don't have a Gmail and a Facebook. I'm guessing that the true proportion lies somewhere between the extreme situations we're in.
Most people associate facebook with procrastination\lols.
More people prefer to procrastinate\lol than to work.
Therefore more people will prefer to Facebook than to Buzz.