The Yammer documentation heavily pushes the use of activities in Yammer apps. That makes sense, as almost any app has some activity that they can push.
However, if your app has comments or attachments, and these are attached to some sort of object, they're a perfect candidates for integration with the Yammer message system.
The advantages of using Yammer messages over activities (where appropriate) include:
- Messages appear in the main Yammer window, as opposed to the smaller activity feed.
- If messages are posted to OG objects, then they are grouped there, making them more useful to your Yammer-linked customers.
- Attachments in messages will appear in the Files area of Yammer. Activity OG objects, even those with type "file", will not.
If you do decide to use messages in your integration, keep in mind that by default messages will be posted to "All Company" if no group or user is specified. This is not a good default in most cases, and it is recommended that you add a mechanism that allows users to specify a group to post to.
It is also recommended that when integrating with the Yammer message system that you let the user know somewhere that they will be posting to Yammer as well.