Sysomos looked at 1.2 billion Tweets over the last two months and found 71% generated no response whatever. Some 23% generated a reply, but a very small percentage (6%) were deemed worth sharing with one's followers upon receipt.
And perhaps the more startling finding is how Tweets wither on the vine quickly --- if they're not ReTweeted in the first hour, they tend not to be at all. Some 92% of ReTweeting took place in the first hour, Sysomos found. (It is possible that Twitter streams are so vast that users can't keep track of what they're sent, so they don't dig very far back to look for content to share.)
How deep are conversations on Twitter? Sysomos said not very. The number of Tweets three levels deep --- that is, those that are sent, replied to, replied to again and replied to again --- amounts to about 1.5%.