[Tori]

@onyx808 you’re a life saver! That code works perfectly.

[Tori]

@intimez I’m not using that plugin, but I did have the same issue.
I hope you’ve fixed yours by now, but if not, here’s what I did:

In the bp-activity-template.php file, edit this function

function bp_get_activity_comment_name() {
		global $activities_template;

		if ( isset( $activities_template->activity->current_comment->user_nicename ) )
			$name = apply_filters( 'bp_acomment_name', $activities_template->activity->current_comment->user_nicename, $activities_template->activity->current_comment );  // backward compatibility
		else
			$name = $activities_template->activity->current_comment->display_name;

		return apply_filters( 'bp_activity_comment_name', $name );
	}

The last line is all you need to change. Basically, you’re changing “user_fullname” to “user_nicename”.
I know it’s not ideal to edit bp files, but this is currently the only way I know how to display usernames in comments. Hope this helps someone!