Facebook started in 20o4 and became pretty popular when they open up their members to non-college students. Twitter started in 2006, but increased interest at a much later time. While Facebook may have a larger member base, probably higher than Twitter, to determine the more popular platform, it also depends on how active people are in each platform and the person's needs in a social platform. Sometimes people make social accounts but no longer use them, which is something to consider. And sometimes your target audience is just on a particular platform but not the other.
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