I love pessimist.
Unless you been living under a rock for the last 2-3 years, you would see that Facebook's growth is far larger then a destination. In a lot of circle it's has become a means of communication a step above email.
It stopped being used as a Social network "per se" a few years ago, now it's more of a communications channel.
I would call it evolution. Smoke Signals, Mail, Telegraphy, Mail, Radio, Telephone, Cell Phone, Email, Instant messaging, Video conferencing, now Face-booking.
People use it as their main median to communication with their large network of friends, associates, and Now Whole families.
It's less of a destination, and honestly more of a dependences now days.
People are kind of locked it for now, with their data stored on it (Pictures, Videos, Connection, Contact, Messages, etc).
I'm a big email guy, but lately i find it easier to reach people through Facebook now.
And with marketing Dollars flooding in, it will get bigger, and make it harder for a replacement! Any potential real competitor can now be purchased in Whole and added.
But you probably hate Facebook, don't believe in it, etc, but just sit back and watch!.
I will bump this thread in a few year ;)