BuddyPress.org

My recollection is that “Last Active” Blogs are defined as those that have the most recent POSTS… not most recent COMMENTS.

It would be a cool option (someday) to select on the back end how you want the Blog Directory to display (“display by last active (X) post / ( ) comment”). For now it would probably be easy for someone who knows BP to write a custom filter that lists blogs according to most recently commented upon.

That said, you can get an Activity Stream listing of latest blog comments. If you are using the bp-default theme, it’s in the pulldown menu under “Show Blog Comments”, if that helps?