I think they were not first-time CEOs exactly. They had entrepreneurial personality as they did small projects before making it big. Newbies are usually those who are trying their hand in entrepreneurship for the very first time.
Agree on what you said about prior experience. But that was not true for everyone. I think Zukerberg has no prior experience. Another example is CEO of Snapchat.
But yes we can say that Mark was little lucky that he got company of Sean who already had hand on experience of building something like that.
Mark Zuckerberg did not have CEO experience but I believe he along with his dormitory friends at Harvard were always talking about ideas and Mark would actually start working on those ideas.Facebook itself was his pet project long before he met Sean or decided to start a business. Though, with Sean's experience in social networking business helped a lot. P.s. Snapchat CEO spent about one year on a college application management software but the product did not take off. :) Here: http://www.techjuice.pk/9-entrepreneurial-lessons-from-evan-...
