Online gambling is a business that's as widespread as it can be in its current legal situation. In many jurisdictions, it is well regulated, allowing their residents to play top quality casino games online at Royal Vegas or the online gambling venue of their choice. In some, it is in a "legal gray zone" - the regulations in force prohibit the Royal Vegas, for example, to promote its services to locals but there's no legal restriction on the locals to use the Royal Vegas' services - Canada and Australia are jurisdictions of this type. Even here, there would be a public for real money online gambling services but the government doesn't allow operators to enter these markets. Finally, there are the jurisdictions where online gambling is outright banned - here, players either fall back to small and brave operators still accepting them as players or choose to play their favorite games through social networks, with virtual money. The USA is the best example for such a jurisdiction.
The almost uniform ban on real money online gambling in the US has created an opportunity for social game developers. Companies like Playtika, for example, have launched insanely successful - and, of course, profitable - Facebook games and mobile apps that offer players the possibility to play casino-style games (slot machines, blackjack, and others) with no real money bets involved. These apps don't fall under the same regulatory obligation as real money gaming venues - they don't involve real money bets - but they offer an experience similar enough to the real thing to make them desirable to the crowds.
The above-mentioned Playtika, a company based in Israel, was among the first to launch a casino-style social game to Facebook (as it was the only social network at the time to include the possibility of playing games inside it). Its games were a great success and later transitioned to the ever-growing mobile gaming space. The success of the company's games attracted the attention of the big players in the real money casino industry - the company was acquired by Caesars Entertainment Corporation in 2011, for an amount between $80 and $90 million. The company's growth continued - its flagship title, Slotomania, has become one of the most successful social casinos ever, and has maintained its position even today. To this day, Slotomania is constantly among the top grossing iPhone apps in the United States, with daily revenues often exceeding $200,000. Playtika was later sold by Caesars Entertainment to a Chinese consortium (which includes Alibaba billionaire Jack Ma's Yunfeng Capital).
There are several other similar apps on the market, including IGT's DoubleDown, Big Fish Games' Big Fish Casino, and several other contenders. And casino slots are quite popular among mobile games in general: according to Verto Analytics, they were the fifth most played mobile game genre in 2016.