BuddyPress.org

A user can only “favorite” the activity record for that new_blog_post or new_blog_comment made within the activity stream part of buddypress – when a user views the main blog that activity information is not pulled in. (i have an experimental plugin which replaces all of the blog comments with activity stream)

Though it might be possible via a plugin to pull in and add that functionality to the blog portion (basically find the _ID() and comment_ID() and look it in the activity_records)