Twitter is rolling out Promoted Tweets in the stream. Good for them. I doubt mainstream users will violently rebel against such relatively inoffensive advertising, even if the digerati went nuts over the notorious – and, truth be told, poorly executed – “Quick Bar” overlay in its mobile app. Twitter will use a biddable marketplace though it’s no AdWords yet. Twitter claims it gets double-digit click-through rates on this type of format, which may be due to novelty as much as “engagement.” Here’s how Forrester marketing analyst Sean Corcoran thinks advertisers should use the new units. He seems overly conservative. It’s good to see Twitter working on how social media marketing can work without driving users crazy. This is an experiment worth watching closely, as real-time stream advertising has barely gotten off the ground.