Don't forget (and this is probably more important in this case than usual) luck. The nature of social networks favors one network that "everyone is on". Which one gets chosen probably has more to do with luck than the execution of the site. All it took was a few random people to stumble upon it and "but all my friends are on facebook" took it from there.
If you'd like a counterpoint that favors execution, I hear that you can do pretty well just selling shoes...
I believe personality is more important then luck, although some things are just lucky (pentyoffish.com) but people who have great personalities, you just like them but you don't know why, they usually succeed.
If you'd like a counterpoint that favors execution, I hear that you can do pretty well just selling shoes...