The real problem would be network effects. Nobody uses a new social network, because nobody they know uses it.
To get people off of Facebook, you'd have to find a way around that.
Yep. You need a niche. Like yik yak, but not.
Integrate with Facebook. Don't force people to choose one system or another. Then people will diffuse across the barrier.
use Facebook oAuth to log into a facebook clone? Will they let that happen?
A CEO that says "no" to an $X billion acquisition.
Start by making a list of cons. What do people not like about facebook? What's the problem with using facebook? That's where your answer lies.
Most people don't have any problem at all with Facebook. We're in a very small echo chamber here on this thread.
Someone with the talent and drive to create a billion dollar closed social network that eclipses Facebook.
What about a for-profit open-source social network then? Reading through comments on threads here it seems the events pages make it difficult for people to switch somewhere else without having a large part of their social life cut off.
It would have to be something that makes facebook irrelevant. For example: a telepathic group mind meld - where you could interact with other people synchronously or asynchronously.