There's a lot that goes into it. Before Facebook was Friendster. Which failed spectacularly because they tried to have some sort of n-squared graph of friends that took thw whole thing down. What FB got right in the early days was it didn't crash. We take that for granted now in the age of cloud everything.

Also, there was Classmates.com. A way for people to connect with old friends from high school. But it was a subscription service and few people were desperate enough to pay.

So it's wasn't just the idea waiting around but idea with the right combination of factors, user-growth on the Internet, etc.

And don't forget Facebook's greatest innovation - requiring a .edu email to register. This happened at a time when people were hesitant to tie their real world personas with the scary Internet, and it was a huge advantage: a great marketing angle, a guarantee of 1-to-1 accounts to people, and a natural rate limiter of adoption.