Restrictive social-media policies have cut off one of the most important advertising routes for marijuana businesses and forced the creation of a new social space, MassRoots.

Isaac Dietrich, seeing that establishment social-media companies shun cannabis users, helped create MassRoots, based in Denver, to fill the gap.Although social attitudes and laws surrounding marijuana have changed, social-media rules have yet to evolve. While a patchwork of states has approved forms of medical and recreational use, federal law still prohibits marijuana, prompting social-media companies to restrict advertising and accounts, even from state-sanctioned firms.

“[Mainstream social media] quite frankly don't understand the cannabis industry,” said Dietrich, co-founder and CEO of MassRoots. “They view all marijuana-related content as if it’s heroin, as if it’s another drug that’s illegal under federal law, a drug they potentially could get in trouble over.”

Every day, marijuana businesses have their Facebook and Instagram accounts deleted.

“It’s ridiculous,” said Nadeem...