The Electronic Entertainment Expo, the annual video-gaming shindig better known as E3, has an undisputed star this year: Fortnite, the online multiplayer game that has blown into a veritable pop-culture phenomenon. After only one year on the market in its latest form, Fortnite has attracted 125 million players around the world.

Two big reasons Fortnite is a hit: It doesn't cost a cent to play, at least initially, while the game and format are addictive. In the game, 100 players are dropped onto an island and have to fight one another until only one is left standing (think "The Hunger Games.")

"The key thing with Fortnite is that it's free, so that enables them to pull together [a] big audience very, very quickly. I think that free model is what you're going to see other people imitate," Ben Howard, Gamespot's vice president of programming, told CBS News.

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Epic Games, the company behind Fornite, makes money by charging players for additional items like extra moves or different costumes. Players also can purchase a "Battle Pass" for $9.99, which lets them complete challenges to earn rewards.

Those sales can add up: In April alone, the company raked in nearly $300 million, according to CNET.

So-called social games like Fortnite are only expected to become more popular, according to gaming experts, and rival companies are creating their own version of the popular product.

"Two of the biggest games companies right now -- Activision and Electronic Arts -- the two big games they announced this week, 'Battlefield' and 'Call of Duty.' have battle-royale modes, which is really the type of game that Fortnite is," Howard said.

Social play is also helping propel broader growth for the video-gaming industry, whose $40 billion in annual revenue makes it bigger than the movie business in North America and the music or sports industry.