New episodes of the Netflix series “Stranger Things” dropped in the wee hours of the morning Thursday. Fans have been waiting more than a year for the retro sci-fi hit to return to the small screen for its third season.

In the meantime, cryptic product tie-ins for the show have been everywhere in recent months, including: a line of clothing at H&M, branded with the fictional Indiana town where the show is set; retro Levi’s dad jeans; classic Nike sneakers; and a traveling fun fair featuring carnival games and a Baskin Robbins truck.

“It was handing out ‘Scoops Ahoy’ ice cream because of the ‘Scoops Ahoy’ store they’re introducing in the third season,” said superfan Tanner Costa who attended the event in Santa Monica, California last week. “It got me 100% more excited for the show.”

These kinds of subtle, immersive campaigns can be very effective, said Barbara Kahn, a marketing professor at the Wharton School at the University of Pennsylvania. Because they feel less like advertising and more like an extension of the narrative, they feel organic and authentic and become very shareable.

“People enjoy it. I think you’re in on the story. You’re in on the joke,” she said.

Synergy between Hollywood and brands predates social media, said Jason E. Squire, editor of “The Movie Business Book.” The classic example is “E.T.,” a film that clearly inspired “Stranger Things” and famously featured Reese’s Pieces after M&M’s declined.

“What a mistake that turned out to be,” said Squire. “The sales of Reese’s Pieces skyrocketed.”

Even a forgotten product from the ’80s is getting new life from “Stranger Things”: Coca Cola has reissued cans of the failed “New Coke,” featured as a plot point in the show.