I've wondered this as well - specifically regarding Fridman but also other people who have become popular through social media. (As to the latter, it never ceases to shock me how various YouTube channels can amass hundreds of thousands of subscribers.)

Anyway, as someone mentioned above, he was on Rogan's podcast and fits the Rogan mold of influencer: interested in AI and machine learning, politically neutral, trains in Brazilian jiu jitsu, etc. In fact, I'd describe his style as being a more intellectual Rogan, haha (which is not necessarily a compliment). His popularity has skyrocketed after being on Rogan. However, I'm not sure about his popularity prior to Rogan.