Facebook is the king of in-the-river promotions - perhaps because they roll out so many new features, promotions, toggles, etc and it's the most effective way to alert users of changes. Here is a great example by Twitter and it's worth showcasing for three reasons:
1. It is a balance of really obvious but in-line. The promotion box sits atop the feed and therefore feels natural - as opposed to an overlay of some sort. Nevertheless, it cannot be missed.
2. It is not done to promote a new feature. It is done to message active users. Better than an email.
3. The focus is on ensuring accurate email addresses and user data. The best to do that, per point #2, message active users in line.