Facebook figured out that using your real full name in an electronic social network had huge advantages in terms of connecting your real world social network.
They killed Myspace because of the network effect the above had when people started finding their friends from high school easily online. Early on I think you even had to join a network specifically for your high school so everyone had their first and last names along with their labeling for their high school.
It sounds stupid and obvious now but before Facebook the average person didn't broadcast their first and last name on the internet.
Right thing¹, right place, right time, executed well enough.
[1] whether this thing was an act of genius or just the right thing if several thrown at the wall, we could possibly debate.
I wouldn't even go that far: it was merely executed well enough that it didn't squander being in the right place at the right time by being terrible.
This is one of the reasons why getting a minimal viable product out quickly (where the definition of “minimal viable” includes that not-being-terrible caveat) can be so important when working on a new idea.