Google rarely advertises anything on the Google.com homepage. Most of the time the ads consist of single-line text banners about a new Nexus device or donating to disaster relief. They were the subject of much criticism in 2012 when they used an animated advertisement (for the Nexus 7) on the homepage for the first time.
Google is once again using its influence as the homepage for millions of users - this time to promote Duo. If you visit the Google homepage at the time of writing, you can see a text banner that reads, "Face-to-face with just a tap. Try Duo, simple video calling for Android and iOS."
Google appears to be making a large marketing push for Duo, just recently they published a series of advertisements on their YouTube channel. But if they want Duo (and later Allo) to take market share away from iMessage, WhatsApp, and Facebook Messenger, they're going to have to push harder.