MyBlogLog was a way to engage with the users of your site. You could install the sidebar and invite people to join. Then you could see who visits your site, or any other site with MyBlogLog installed.
The API was used to query the MyBlogLog social network and pull meta-data associated with user and site profiles. Developers could access the MyBlogLog data programmatically.
Yahoo shut down the service in May 2011.