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Facebook's metaphor is the high school yearbook, where you post photos and comments about yourself and your friends. It is not well suited to dividing up who you know and who is allowed to see what -- far too many stories abound of Facebook users losing their jobs because of comments that they posted for the general public to see.
The Google+ metaphor is similar to a Venn diagram of who you know, how they fit into your life, and can help you to keep different conversations private from different groups of users. Circles of friends may not necessarily be welcome mixtures amongst your circles of work acquaintances.
Google+ also provides a mechanism for communicating with them in REAL TIME. Whereas with Facebook you have discussions, in Google+ you can create ad-hoc video conferences with several people at once. Apparently that's something that Facebook is hoping to address by adding Skype support. I haven't tried Skype on Facebook but I'm looking forward to that.