Also the "average" person won't maintain 2 social profiles and will gravitate to where the users already are.
The MySpace to Facebook transition shows that exactly this happened. I was in high school at the time, and I definitely remember people mentioning having both a MySpace and Facebook account, and wanting to get people to switch over to Facebook. I don't mean "tech people", I mean normal folk.
As for compelling features, I think Circles is a killer. When your relatives are on Facebook, a good portion of your personality has to leave Facebook. You have to think about what you're going to say, and generally stick to the blandest possible expression of who you are. If Google+ enables people to say what they want to who they want, it will be a hit.
Short term there may be some cross over, as with MySpace and Facebook, but long term the average person will only maintain one personal "social" profile ie I don't include linkedin(business) or Twitter(broadcast/news) as a personal social profile (I know me not including twitter as social is asking for trouble but its a different kind of beast). MySpace is effectively dead after being sold today for a 10th of the price News Corp bought it for.
Facebook already has the exact same functionality in friend lists. They're only a small ui tweak away from parity in that respect. As for the drag and drop into circles, it's cute but realistically it's not a big part of day to day use.
The MySpace to Facebook transition shows that exactly this happened. I was in high school at the time, and I definitely remember people mentioning having both a MySpace and Facebook account, and wanting to get people to switch over to Facebook. I don't mean "tech people", I mean normal folk.
As for compelling features, I think Circles is a killer. When your relatives are on Facebook, a good portion of your personality has to leave Facebook. You have to think about what you're going to say, and generally stick to the blandest possible expression of who you are. If Google+ enables people to say what they want to who they want, it will be a hit.