It's much too soon to know what the effect of Google+ will be on Facebook or other social networks, but I'm betting there are lots of people who may transfer their social conversations to Google+ if it is and becomes as open as it seems to be.
There's an excellent article on the O'Reilly site, explaining Google+ and speculating on how it might be used. I recommend it.
The issue is that a social backbone, supporting many kinds of social interaction, must be open. Users must control who gets what and be able to take their data elsewhere if they choose. Facebook doesn't work that way -- but Google has no need to be a walled garden.
I can't wait to see what happens next.