It's not MBA mentality, it's stock investors mentality. I don't think Zuckerberg or any other executive is out there buying FB shares at $320. It's outsiders gambling on the continued growth.
Share buybacks usually indicate the company can't figure out what to do with their cash stockpile, and are doing a tax-advantaged dividend (it effectively gives the money to shareholders, but they don't have to incur a tax hit).
It could mean they're really certain it'll be worth more later, or it (more likely) could mean they ran out of ideas and are trying to shore up the stock price, too.