When Mark Pincus hired a new executive to run Zynga, the online game company he founded, he wrote on Twitter, calling the executive, Don Mattrick, an “Internet treasure.”

Now, less than two years later, Zynga’s Internet treasure has left the company, and Mr. Pincus has returned as chief executive.

The departure of Mr. Mattrick was not altogether surprising to many in the industry, since a long-running turnaround plan that he set in motion at Zynga had yet to take flight. The return of Mr. Pincus to the top job, though, was unexpected since he had seemed to have largely disengaged from the business of running Zynga, best known for early Facebook games like Zynga Poker and FarmVille.

The abrupt change in leadership was another setback for a company that was once poised to be a leader in a new era of games and the Internet. Other Internet darlings of the same era, like Groupon, have also faded after their growth fizzled and profits proved elusive.