Four to Five rounds of interviews.
One round was based on puzzles and guesstimates.
Other rounds were based on game design and creativity, ideas for new features etc.
Playing a couple of games in detail will help.
I applied through a recruiter. The process took 1 day. I interviewed at Zynga (Bengaluru) in Apr 2014
Interview
The interview process was a very good one with good PMs interviewing me. There were 4 rounds and all of them went smoothly. First two rounds were technical in nature, with the last two rounds being cultural fit rounds.
Interview questions [1]
Question 1
They asked me to solve a few puzzles and my knowledge about social games