Subscriber count still excludes social media followers on wp-admin/admin.php?page=stats

Brought to my attention in 640099-zen.

In #800 we finally updated the count to include social media followers, for the Subscriptions widget in particular. However, we have not updated the wp-admin Stats page to also include the social media followers, which can lead to confusion.

Start at wp-admin/admin.php?page=stats and compare the Stats-page subscriber count with the social-follower change described in issue #800. Trace the existing count calculation and verify that the Stats page includes social media followers consistently with the Subscriptions widget; confirm the resulting count in the admin page.

Written by the indexing model from the issue text.