The leader-boards are a way for you to see your ranking for reputation within your groups and contacts,
Your Global Ranking will only show if you are in the top 10,000 and compares you to every Palringo user in the world. Global will not show if you are not in the top 10,000.
Your Groups Ranking compares you against the Palringo users you share a group with.
Your Contacts Ranking is your position among your contacts.