Pretty sure Facebook took off because they required you to have a .edu address, and even then, when it first launched it wasn’t just any .edu, the rolllout was slow. I remember people enrolling in 2-year schools when the regex matched on *.edu just so they could get an account.

Facebook hit the seam of internet 2.0: after the .com crash with a bunch of kids who grew up on AIM/ICQ/whatever and all these kids wanted to keep up with their friends at various colleges.

They indeed just got lucky.