To be fair, Zuckerberg is a bigger target than most people, but that wasn't the real reason his social media accounts outside of Facebook were compromised. It's because he was lazy with security, both with the password he chose and in reusing the same one for multiple websites and services. That's a major no-no if you care at all about staying in control of your accounts.
The hacking group OurMine Team took responsibility for the hacks, though it wasn't a sophisticated security breach by any means. All they did was comb through the data from the LinkedIn password dump from a few weeks ago, find Zuckerberg's login details, and try those same credentials on other websites.
Ouch. Mark Zuckerberg's social media accounts have been hacked pic.twitter.com/KvVmXOIg5s— Ben Hall (@Ben_Hall) June 5, 2016
The affected accounts have now been secured. Furthermore, Facebook issued a statement saying that none of its accounts were accessed or affected by this incident.