The number of Americans who use Facebook will reach 169.5 million in 2018, making it the most popular social site among all ages, except for teenagers, who prefer Snapchat, eMarketer reports. Some 16.4 million Americans between the ages of 12 and 17 will use Snapchat this year, compared with 12.8 million who will use Instagram, and Snapchat will remain the most popular social site for teens.