After analyzing over 1.2 billion tweets, the Sysomos team found that only 29% of tweets actually produce a reaction - that is, a reply or a retweet. According to Sysomos, just 6% of all tweets are retweeted and these retweets have a very short lifespan. Virtually all retweets happen within the first hour after the original tweet.
(via The Short Lifespan of a Tweet: Retweets Only Happen Within the First Hour)
