Business Insider talked about how Zuckerberg doesn't do angel investing because it would take his focus off of his core business - Facebook. He doesn't feel like he can do his job with Facebook and spend time on other ventures. This was also true with Steve Jobs and Apple - he didn't do angel investing.
Then I started thinking about how this should impact me in my life - what is my focus. During work hours my focus should be 100% on building a great business that will be around for the next 100 years. During the day what do I do that takes my focus off of business? Do I allow non-work things to sneak in and take time?
Staying focused is a difficult discipline because there are so many distractions but the payoff is huge. If it's good enough for Mark and Steve then it's good enough for me.