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Facebook wasn't always the 500lb gorilla, it was once the new kid and MySpace was the established social network. And where is MySpace now? Dunno, haven't bothered to look in a couple of years. Facebook isn't likely to fade away like MySpace, but the point is that what looks like a dominant player now may not be in a not too distant future. Google+ may not be nearly as popular as Facebook at the moment, but there were about 50 million Google+ visits last month, which is a pretty good start. http://venturebeat.com/2012/01/03/google-plus-traffic-high-december/
Google+ is appealing to me in some ways that Facebook is not. First of all, while Google may not be perfect as far as privacy issues go, Facebook has a terrible privacy track record. I also like how simple and intuitive the "circles" function is on Google+, it makes it much easier to keep online life segregated between groups of people. Facebook has this feature too, but it isn't as easy to use. Being a late arrival gives G+ one more appealing feature to me; I won't stack up "friends" like I did when first on facebook, there will be some winnowing out this second time around.
The big thing from a business perspective that Google+ has going for it is that it allows you and your friends/acquaintances/whoever circles to influence each others' google search results. The google+ button that you see on many websites not only allows you to post an article to your wall, if something is interesting but not "wall worthy" you click the button without sharing with your circles, and if the website is appropriate to a search by someone in your circles it may pop up in their search results with a tag noting that you +1ed it. I can see that as a very positive feature for businesses using Google+ for social media marketing.