I'm not suggesting that FB is the only time waster. As you point out, there are many avenues for that. My main point was comparing Zuckerberg to a few other people that have amassed vast fortunes. The others, not without their flaws by the way, at least created something of great value. Something enduring.
What has FB created? Yes, there have been some positive benefits such as people locating long lost friends, etc. But you have to dig pretty deep to find them and it was probably more by accident than by design. Sorry but clicking the "Like" button does not qualify as a boon to mankind in my books.
Even if you look at people that were not rich - Hemingway, Shakespeare, Mother Theresa, Isaac Newton, Alexander Fleming - each of them contributed to the betterment of the human race in their own ways. Contributions that far outlived them.
What will become of FB in 20 years time? Sure, it's all the rage right now. But how long until some shiny new thing comes along? How long before people get bored of clicking Like?