Here's a chart of social media usage from Pew Research. YouTube and Facebook are by far and away ahead of the pack, but Facebook's been stagnant for a few years, at least in the U.S.
Facebook and YouTube dominate this landscape, as notable majorities of U.S. adults use each of these sites. At the same time, younger Americans (especially those ages 18 to 24) stand out for embracing a variety of platforms and using them frequently. Some 78% of 18- to 24-year-olds use Snapchat, and a sizeable majority of these users (71%) visit the platform multiple times per day. Similarly, 71% of Americans in this age group now use Instagram and close to half (45%) are Twitter users.
Note that there's no tracking data on YouTube: they only just sampled its popularity in their surveys. Like everyone else, they only just noticed that it was Google's real social network all along.