So if there was no Facebook would another Facebook clone take it's place? Intuitive answer is yes, until you start thinking. How did Facebook got so big? It was best product on the market. So now Facebook is dead, what happened to Facebook? Is there a reason it died, if Facebook could not survive, how will it clones will not only survive, but thrive?
So here I would like to make a small prediction. The only way there will be another Facebook, if someone would make something better. Something which will either do things that Facebook does better - and hence kill it off, or something that will open entire new market.
TL:DR If you want to get big you can't be another Facebook: you either must be different or better.
How many here remember sixdegrees.com? (1996) That's the first social networking site I remember signing up to...
Of course you must be "different", but the difference between each successive evolutionary step from Six Degrees to Facebook has been tiny.
While it is getting harder as the size of the successive networks have gotten bigger (SixDegrees reached a low number of million users), because of the network effects, it's not so much about features as about finding a way of building the network to a tipping point, and social networks are incredibly fickle and subsets can migrate to new services because they are not the dominant network.
People like to be unique. They don't like to be grouped in with the rest of the world. This is especially true with teenagers. You don't have to make a better Facebook, you just have to make a new Facebook. It needs that feeling of being fun and exclusive, which Facebook is quickly lacking and can never gain.
Google+ will never succeed. Yes, they've invested countless dollars and time from some of the brightest minds available, but the name Google is simply too big, and too well known. It's boring from day one, there's nothing cool or hip about signing up on Google+.