Yet another new service has been introduced in a slow roll-out on Twitter. The microblogging company has added a much-demanded retweet function, copying the original tweet under the user’s byline when they retweet. The downside is that users no longer get to say anything about the tweet they quote, and it could insert unwanted users into retweet streams. Confused? Probably. But it’s potentially very valuable for Twitter: the new feature enables metrics for tweet ranking, providing free topical databases that can be sold off to companies. Hopefully for Twitter, everyone won’t just turn it off.