Using WhatsApp online status and last seen features can allow someone to analyze the figures and get a good idea of a user’s sleeping patterns. The last seen settings allow everyone to have access to the data, restrict them to be seen by contacts only, or allow them to be hidden to everyone. By default, this information is available for anyone to see. The online status information cannot be hidden or restricted in any way.
Knowing the sleeping patterns of messaging app users is the kind of information that online advertisers would treasure, so this vulnerability could, in theory, help launch a money making venture for someone.