"When a tweet generates a reply (aka @) or a retweet (aka RT), it suggests the tweet has resonated enough with someone that it sparks a conversation or encourages someone to share it with their followers," the company writes. http://sysomos.com/insidetwitter/engagement/
Of course this doesn't consider that lots of people read or view content — on Twitter and in other forms of media — and don't necessarily feel compelled to respond.
According to the Sysomos study, just 6% of Tweets measured got retweeted and 23% were apparently intriguing enough to warrant a reply."
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