Mark Pincus, founder and chief executive officer of Zynga Game Network Inc.

Mark Pincus is the latest social-media CEO to become a billionaire – and then a former billionaire – in just six months.

This spring, when Zynga shares were trading above $14.50 a share, Pincus’s 94.5 million shares were worth more than $1.3 billion. Now, his shares are worth less than $250 million.