Why Are Tweets 140 Characters Long?
When Twitter launched in 2006, the intention was that a substantial subset of users would post their updates via SMS text messages from their phones. An SMS has a 160-character limit; any longer, and the message is split over multiple texts. To keep users' phone bills down, Twitter engineers capped Tweets at 140, which would also leave room for a user name and colon in front of the Tweet.
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