The microblog portal is the on place where microposts, advanced search and settings are located. You can get there by clicking the link in the application header.
The microblog portal is also available under: http://<Confluence-URL>/plugins/microblog.action.
If you updated to 4.0 some functions moved from the macro timeline sidebar to the portal. This includes the settings for "My Spaces & Topics", E-Mail notifications and the advanced search.
Self Service - Microblog Placeholder Macro
If you use some sort of template mechanism to create new spaces (requires other plugins) or space blueprints with a Microblog Timeline, this feature makes it more easier to start microblogging right after creating the new space.
Inserted in one of the template pages, it will be converted to a Microblog Timeline with the newly created space selected. Furthermore it creates a default topic with the name of the new space.
This is the Microblog Placeholder macro in a template space.
This is not a real timeline, but an image of one.
After copying the Template page with the Microblog Placeholder macro, it is replaced with a Microblog Timeline macro which only shows microposts from the current (newly created) space. As you might noticed there was a default topic created which the name of the current space.
You can insert the placeholder macro within the Confluence Default Space Template, so every new Confluence Space contains a microblog timeline and topic.