Facebook made a name for itself by starting as a closed network available only to Harvard students initially and then branched out from there to other college campuses. It gave an air of exclusivity to the network and gained critical mass through that approach. After they had reached a stage where they could no longer be ignored they opened the floodgates and allowed anyone onto the network.
Many Facebook users have already grown tired of the network, and having Mom, Dad, Grandma and Grandpa view every update of your drinking binge or dirty joke certainly hasn't helped the feeling of being overwhelmed with your information being shared with everyone and no easy way to fix it.
Google+ is banking on a totally new way of sharing information and giving you an easy way to decide who get's to see exactly what... without making you jump through hoops to do so. Facebook would have to turn their entire culture upside down to accomplish this and it goes against everything they've said they stand for in the past... the open sharing of ALL information.
Early adopters can make or break a social network, and Google+ seems to have attracted and won over a huge percentage of very valued early adopters. I'm talking about Hacker News users and their near total preference of Google+ over Facebook.
As seen in this poll: Hacker News | Poll: Google+: Like it/prefer it to FaceBook, Google+ has over 1023 votes that like it and prefer it to Facebook, vs 160 votes that prefer Facebook over it.
That's a huge margin of victory for a beta product that is still very tough to get into and does not yet fully support profiles for Google Apps. If I'm a Facebook stock holder at this point, I would be very worried. We all know how fast a large social network can fall from grace. Take a look at Myspace - from $580 million to $35 million over a course of less then 6 years. And in reality, they were pretty much in the coffin after only 2 years.
Now I don't believe that Google+ will kill Facebook but I do believe that it will steal a large amount of market share and prevent Facebook from taking over. It will make Facebook honest with how it treats app developers and users, much like Android has done to keep Apple honest and more open.
This is good for consumers and good for the Internet as a whole.
As an aside note, we have put up an entire site dedicated to Google+, you can visit it at this URL: Google+ / Google Plus Forum
(Source: Facebook should be _very_ worried)