I am using the connect API to provide a visualforce page with feed items for an Account. Unfortunately, my community user seem to only be able to view his own posts, not ones made by the internal users. I know I am probably missing a setting and my face is ready for palming, but what am I missing here?
When an internal user posts a feed item to an Account (or any other type of record), there's an option to control the visibility of the feed item. The options are to make it visible to internal users only, or to make it visible to all users who have access to it. The default is to make it visible to internal users only, so that community users don't see the post. That's probably what you're running into. Here's where the option is in the Salesforce UI:
If you log in as an internal user, you can get the details of the feed item via the API and then inspect the "visibility" property of the ConnectApi.FeedItem to see if the theory's correct.
When you post a feed item via the API, you can set the visibility property in the ConnectApi.FeedItemInput class to achieve the same thing that the visibility drop-down in the UI does.