American politicians seeking to do well on social media may want to take note: one of the world’s best will be in New York this weekend.

Narendra Modi, the Indian prime minister, will address a sellout crowd of supporters at Madison Square Garden on Sunday, and he’ll do so not just as the leader of one of the world’s largest countries but also as a juggernaut of political social media.

Take a look at the raw figures: Among politicians, Mr. Modi ranks second behind only Barack Obama in number of fans of his official Facebook page (Mr. Modi has 21.8 million and counting). No other political leader is even close. His Twitter account and that of his office are among the fastest growing among politicians and elected officials worldwide. Among public figures who have some political sway, he trails only Mr. Obama, the Dalai Lama and Pope Francis in Twitter followers, with 6.62 million.