Can you clarify what the Gnip is:Retweet rule means in the data retrieved from Sifter and DiscoverText? Is the is:retweet metadata field set to true when a user presses the RT button? Is it only the first Retweeter that counts?
A Retweet is a re-posting of another Twitter user’s Tweet. The Twitter Retweet feature helps you and other users share that Tweet with all of your followers.
Retweets look like normal Tweets with the author's name and username next to it, but are identified by the Retweet icon and the name of the user who retweeted the Tweet.
Some people type RT at the beginning of a Tweet to indicate that they are re-posting someone else's content. However, this isn't an official Twitter command or feature, but signifies that they are simply quoting another user. If desired, you can search for the presence or absence of the letters “RT ” in a Tweet, but keep in mind that this is not what Twitter considers an official Retweet.
Twitter will tag a Tweet as a Retweet (and DiscoverText will add in the metadata item for the is:retweet rule if it exists) if the Tweet itself is an actual Retweet - that is, only if the user used the Retweet button on Twitter. If the user copied and pasted the text (which a lot of Twitter users do), then it will not be flagged in the metadata as a Retweet.
Keep in mind that any metadata values are from a point in time when the data was collected. Any live Twitter display is real-time for Retweets and favorites which are dynamic. Therefore, the data in your collection gathered days earlier may differ from a current view of live Twitter data.