The situation is poised to get worse: Spending on social gaming is forecast to decline 7 percent further this year, according to SuperData.
"The market for social gaming is fragmenting and given the numerous interests of advertisers, end-users and Facebook, there may not be much that social game companies can do to improve the current situation," SuperData founder and CEO Joost van Dreunen told CNBC.
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Casual mobile games have siphoned a large share of time and money from the social games market, he said. "Many social gamers have simply moved on to mobile platforms such as smartphones and tablets."
All of which means that Mark Pincus has his work cut out for him as he resumes his post as chief executive of leading social gaming company Zynga, shares of which are down nearly 75 percent since its December 2011 initial public offering.
In van Dreunen's view, Zynga was slow to transition to this new landscape and consequently lost its lead over King Digital Entertainment, maker of the massively popular "Candy Crush Saga" and Supercell, which develops mobile strategy games like "Clash of Clans" and "Boom Beach."
During outgoing CEO Don Mattrick's nearly two-year tenure, revenue for Zynga's top-performing franchise "FarmVille" fell from $9 million a month in mid-2013 to a little more than $2 million a month today, according to SuperData.
In the same period, social gaming's monthly active user base remained essentially flat at about 188 million gamers, while overall social gaming revenue contracted by 6 percent.
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Unlike Zynga, King has managed to offset declines, and its earnings from "Candy Crush Saga" on Facebook are comparable to levels two years ago. That is owed in part to its monetization strategy, said van Dreunen. King lets users get deeper into its games before asking them to pony up money, while Zynga has made gamers pay for premium items relatively early, he said.
One exception to the overall decline in social gaming: casino gaming. Average monthly spending for social casino gamers is roughly double what users of traditional social games like "FarmVille" spend, according to SuperData.