Master P thinks a couple of young guys who are huge online will be mainstream stars in no time ... and his upcoming movie's going to be the catalyst.

We broke the story in December -- P was planning to cast some raw and unknown talent for the sequel to his 1998 movie, "I Got the Hook-Up," and he tells us he's stayed true to his word ... by signing social media stars Fatboy and Piola.

P says Hollywood's failed to recognize the power of social media and its big personalities, and believes by casting Fatboy and Piola -- who have nearly 7 million followers on Instagram combined -- he's connecting with an untapped audience.

To put it bluntly, Master P tells us ... "What we're doing with 'I Got the Hook-Up 2' is genius."

He's got high hopes for his new actors too ... telling us they're dynamic, natural talents with amazing chemistry because they're both great friends. P says he's going to add them to the mix with established actors in his sequel and let the sparks fly ... and has no doubt it will work.