I agree with houstonlively that it seems silly if someone wants to make an exact copy of Facebook. However, there is a market for this. There are a lot of people that would be willing to pay money to make a site like Facebook.

However, every now and then somebody makes a unique spin on the Facebook brand. Take for example Shagbook.com - I saw it the other day and thought it might have a chance of succeeding. It's already on the second page of Google for "Adult Dating".