This is called the reason behind YouTube's decision to add better-messaging features and try to present options other than just the video for its users. Now, Google is reportedly trying to feature more of these features by changing the name of a number of tabs in the app.
Tabs currently labeled as Activity for YouTube are being tested under the new name Inbox, and within this tab, the section labeled Shared will be named as Messages. In addition, YouTube is testing changes from using the word "Contact" to "Friends".
The name change is still in the testing phase, but the changes are more detailed in explaining tab functionality so it's easier to understand when compared to the previous name. All these name changes are part of YouTube's plan to add more social features to the service.
This change begins in a better way to split the video directly from within the app than before, by dividing the link through another messaging service. Then, YouTube adds a social feed for the channel to be able to deliver updated text and image updates to followers.
Until now, these changes have not been ascertained to work properly, but are judged to make the video-based platform more in-going and carry a more social usage experience.