The United States now has the highest social media penetration rate in the world, with a staggering 81% of the population having at least one kind of social media profile. Overall, there are now 208.9 million social media users in the United States, and the social media trend shows no signs of slowing anytime soon: by 2021, the U.S. could have 217 million users. Based on the latest data, here are three key trends to know.