David Marcus, the head of messaging products at the Menlo Park, California company, says in a Facebook post that M can do things like buy items for you, get gifts delivered and book restaurant reservations or appointments.

Apple's Siri is the most well-known virtual assistant. Microsoft and Amazon also have options. Marcus says M aims to do more than other digital assistants on the market by completing tasks on the user's behalf. 

With more than 700 million users, Messenger is one of the world's most popular messaging apps.