That's why I posed it as a question and mentioned the spectrum that this sits on. Facebook got lucky in some ways, I don't think Zuckerberg meant to create artificial scarcity when it launched, I think he was growing it deliberately but the effect was unintended.
Facebook started with Harvard students. Then Boston area colleges, Ivy League, then it was expanded to most universities with a .edu domain. You didn't need invites (Google's preferred method), just to be a member of the right kind of institution. It launched to the public at large in 2006.
Those first couple years, a lot of the use of Facebook was reconnecting with old friends. High school classmates, or if you'd graduated but still had a university email your fellow college classmates. This was a distinct endeavor from Myspace. It was explicitly about connecting people who shared a particular geographic or institutional social context (not any context, school). There were no bands, no autoplaying music. Just you and people you may or may not know connecting to each other.
Now, they obviously intended to grow it into something larger. And they have done a great job with it. Here's the question:
The choice to open it to universities was obviously deliberate, but was the intent to test scaling Facebook or to create an initial critical mass of likeminded, like-aged individuals?
My wager is on the former, though I could be proven wrong. That initial smaller but focused group is what allowed them to successfully compete.
Luck: They stumbled onto a successful model for building a from-scratch social network with their focused approach.
Skill: They intended to be focused on college-aged students believing it would build their critical mass social network more effectively.