None of them had nearly the same reach and discoverability as Facebook does. Good luck finding your childhood friend whose name you barely remember through thousands of web forums with primarily pseudonymous users. Meanwhile, Facebook's recommendation engine will just throw their name at you out of nowhere.

Not true. I was an army brat. It finds nobody in my childhood even though I've tried.

People would have a better chance of finding me via one of my various pseudonym's online than they would by my IRL name.

I do not have any accounts in my real name, and I had no problems finding others via our pseudonym's when I wanted to share them with people in the physical world.