Facebook Inc. Chief Executive Mark Zuckerberg saw his net worth decline by about $5 billion Monday, but it could have been worse. Ahead of Facebook’s worst one-day decline since 2012, prompted by news that data affecting 51.3 million members was improperly shared with a political consulting firm, Zuckerberg had been busy selling stock. So far this year, he has sold nearly five million shares. Disposing of those Facebook FB, -1.02% shares before Monday ended up saving about $40 million, according to Securities and Exchange Commission filings and some arithmetic by MarketWatch. At Monday’s close, the 4.9 million shares Zuckerberg has...