The UFC Store is Americacentric, meaning that, for example, even though Islam and Khamzat are among the world's most popular fighters, their world fanbase is unlikely to pay what is to them exhorbitant prices and shipping fees for UFC store products, and I really doubt that the UFC Store ships to Dagestan, Chechnya, and the rest of Russia. Of course 4 of the 5 most popular UFC fighters - based on UFC Store sales - are American as I'm guessing that the majority of UFC store sales are to Americans.
Who has the most social media followers is more telling, but you'd have to include all social media followers in the respective followers' native languages as well. China, for example, has at least 6 popular social media platforms which are only in Chinese.