He was also lucky, in the right place and time, and savvy enough to exploit his luck. Plus, the value of Facebook (and other sites mentioned) is the community and mindshare. You can't just buy a Facebook clone and expect overnight success.
There's no good reason to believe that Zuckerberg is lucky at all. Saying he was in the right place at the right time has this tacit assumption that if he were somewhere else at the wrong time he would have tried the same thing and fell flat on his face.
It seems because Facebook is an outlier, people feel safe talking about the luck factor, but that's meaningless because we all exist with individual circumstances, and by that measure everything every one of us does is based on luck. Instead, I prefer to ascribe luck to things that the individual actually had no control over, such as winning the lottery.
Being the right place and right time is a huge part of it. He was the right age, in the right environment, when the right technology reached the right tipping point and critical mass.
I'm not disparaging Zuckerberg's ability or hard work. However many with greater ability and who have worked harder have failed because of circumstances beyond their control.
Building a million dollars company is hard work (I mean 10 million at least ), but still billion dollars company are mostly about being lucky and exploiting it. I doubt that billionnaire entrepreneurs seriously worked much more, or much smarter than millionnaires. Billionaire companies landed there because they exploited ideas inherently more monopolistic, more able to spread.
If you take the sivers multiplier http://sivers.org/multiply, ideas still have some potential : a x20 multiplier is a huge boost.
Yes, there is, in the literally uncountable possible paths that the decisions of every other person on the planet (not to mention the state of the planet itself) create, there probably does exist a path to success, if not all, at least most people. Some of those paths involve crime, deceit, coercion and such but that is irrelevant.
"Luck" is you making the right choices (naturally based on your genes, psychology and the decisions that have led you to your current self), others making choices that facilitate it and the planet not blowing up in this process.