They're simple, social, and entirely made for mobile, all of which might explain the explosive growth of standalone messaging apps even as Facebook offers a free way to chat on smartphones.
One of the biggies, Kik says that it now has more than 50 million users and that people -- mostly young people -- are signing up at a rate of 200,000 a day. That's twice the rate that Kik claimed when it last released stats. Just last November, the app claimed 30 million users and said that 100,000 people were signing up each day.
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