The historic drop of Facebook stock.

The social medium saw its share value drop nearly 20% in a 24-hour period late last week—that’s over $100 billion, the most severe one-day loss in stock market history. The apparent reason: a decelerating rate of new active users and soft advertising revenues. Facebook CEO Mark Zuckerberg personally lost almost $17 billion in his net worth. Mr. Zuckerberg, unless and until the stock price readjusts, will have to scrape by on $67 billion.