Zynga headquarters in San Francisco

Not long ago Facebook friends did brisk digital business swapping crops on Zynga's "FarmVille." Now many of those fields are laying fallow as social gaming is stuck in a prolonged decline, even while the overall digital video game industry booms.

Revenue from social gaming apps fell 10 percent year over year to $154 million in March, and monthly sales in the segment are now down 42 percent from a peak in May 2012, according to digital gaming analysis firm SuperData Research. At the same time, overall sales in the digital gaming market rose 2 percent year over year to just more than $1 billion last month.