A post I drafted a few weeks ago and never published... better late than never (I think!) Two good examples of In the River promotions for two new, relatively off-stream product launches: Facebook Messenger

a new mobile application by Facebook focused on Facebook Messages. Location is a highlighted feature that is clearly more important on mobile than on the web. After download, Facebook walks users through the app's key functions - and here is how they alert you of the location feature. Impossible to miss (which is important considering privacy implications, a new feature and a small icon potentially unfamiliar to many):

Twitter Images

Very similar: Twitter released a new product feature (Twitter Images). The product concept is simple and familiar - but the act of sharing via Twitter will be unfamiliar. So Twitter displays a pop-up promotion on Twitter.com that shows the basics, the icon and a link for deeper information. Also similar to Facebook's location alert, Images is an important part of future strategy (see my thoughts here) - but also have privacy and partner implications.... so a clear product launch and description is important: