I will have to disagree with you. Their biggest technological achievement was the Facebook app platform.
While this was indeed an "incredible" thing, the question is, did this help make Facebook what it is today, or even attract a sizable audience?
The answer is most resoundingly no. Facebook was already very popular by the time most of these unique things were made, and in the case of Facebook apps, it even put off some users who were annoyed by its spammy nature.
They haven't done things perfectly (apps), but they did a few things that were enormous, not technologically but conceptually. They had a few incredibly brilliant ideas that made them as lasting and as sticky are they are.
Face book is going to be "dead" (ok, shrinking) in 10 years. If you look at social networks they are only sticky in the short term. Facebook's growth has hidden the fact that the average user does not stick around for 2 years and at 300 million users they just can't keep replacing people. And as people stop showing up it becomes less useful to those who stick around.
The pattern is simple, a new site shows up that let's you exchange info with people you have not talked to in a while. So you sign up and chat for a while, but you don't really care about those people so after a while going to the site becomes an annoying time sink and so you drop it. For a site that just has your core friends. But then you don't talk to people for a while and...
The only long term niche is to become the psudo holiday site, where people occupationally show up to chat with their extended social graph, but you don't really spend much time there. So, lot's of people, few page views.
PS: The people that "love" social sites and want to keep up with all their friends will jump ship as soon as something else becomes popular with most of their friends.
I know I got scolded for this in the other thread, but I disagree, and I'm working on a longer refutation of that "social network pattern." So give me a bit and I'll have a response for you.
Size doesn't matter. Utility does. If Facebook were to start completely over again against the current competition, I think they would still rise to prominence without much trouble.
What makes Facebook different, in short, is that while other services offer a place to talk, Facebook is built as a utility in a way other sites aren't. It's the new big communication medium. The people that use it are using it as a replacement to email and IM. That's a huge deal, because it suggests Facebook has the potential to last as long as email has.
Your suggestion that people abandon things all the time is countered by AIM, which has been the prominent IM service for a decade.
Size matters if you need it to become an enabler. You can't get sick of email and leave, for example. Backpackers are constantly using facebook to keep track of their rapidly evolving social sphere. You can rely on most of those you meet being on facebook. This lets you be confidant that you will be able to use facebook to meet up at a later time or organise to do something at some later point. It doesn't matter if you like writing on people's wall in this case. It's hard to backpack without it. That's a consequence of size.
If facebook is how office drinks get organised, you need ot be in on that. (size doesn't matter so much for that though)
While this was indeed an "incredible" thing, the question is, did this help make Facebook what it is today, or even attract a sizable audience?
The answer is most resoundingly no. Facebook was already very popular by the time most of these unique things were made, and in the case of Facebook apps, it even put off some users who were annoyed by its spammy nature.