Good thing social media did not exist during the era of “Mad Men.” It might have put Don and Peggy out of business. Brands and advertisers, looking for ways to reach audiences beyond television screens and magazine pages, are turning to people with many followers on social media and paying them to pitch products online. The social media stars, in turn, are finding that working as a conduit for a brand can be quite lucrative — sometimes generating more than enough money to live on. Robby Ayala, for example, dropped out of law school to pursue his career as a full-time video creator on the video service Vine, publishing several goofy six-second movies to his 2.6 million followers each day. Last summer, he got a call from Niche, a company that wanted to hire him to make a short commercial for GroupMe, a messaging application, and post it for his followers to see.