Very nice post from Nishant
Was working on a task wherein we had to add Social Pane to a form of a custom entity. We enabled the Notes for the entity (Activities was already enabled).
However after adding it to the form it was only showing only Notes tab.
The solution we found was to export the entity and change the following value to 1 for the particular form.
Saving and importing back the solution fixed the issue.
The helpful post
Hope it helps..