You should head on over to Quora and read this post: “I already use Facebook and even Twitter. Why do I need Google+?”
The question asked in more detail on the site is:
What are some things that Google+ will give me that Facebook (and Twitter) won’t? Is Google + a “new thing” or simply a different way of doing the same thing? – What type of contacts, information, content would I have access to with Google + that I don’t have with Facebook or Twitter today? Like, I’m in StumbleUpon – gives me something that Facebook or Twitter don’t. I’m in Quora, gives me something specialized (Q&A) that’s better than what Facebook does… but what is “the thing” about Google +? I just don’t get it.
It’s a good question to many right now, especially die hard fans of those “other” respected social networks (and people that haven’t as yet got access to Google+).