One WhatsApp user could reach 2.5 million people within seconds via group forwarding.

Evidence

This means that when you share a message with everyone in your local group, up to 256 people will see it instantly and can in turn share it with others. Before WhatsApp implemented counter-misinformation measures, people could forward a message up to twenty times. Assuming someone is a member of twenty groups comprising 256 individuals each, this means that a single individual could expose over 5,000 people (20 × 256) to a message or video in just a matter of seconds. Importantly, these 5,000 recipients could each in turn expose the message twenty times to a group of 256 people. Now we have reached 2.5 million people, and we haven’t even begun to consider friends of friends and their friends. — Sander van der Linden, Foolproof, 334

While those numbers were impressive at the time, what’s unique about WhatsApp and social media, is that you could now instantly share something like the moon hoax with a massive group of people as part of a private messaging system. You simply start a group, give it a name (for example: ‘friends’, ‘family’, or ‘sports team’) and add contacts to the group. When you send a message to the group, all of its members will immediately see what you posted and can, in turn, forward the message to others. There is a limit to how many people you can add to a group. The magic number is 256.‡ This means that when you share a message with everyone in your local group, up to 256 people will see it instantly and can in turn share it with others. Before WhatsApp implemented counter-misinformation measures, people could forward a message up to twenty times. Assuming someone is a member of twenty groups comprising 256 individuals each, this means that a single individual could expose over 5,000 people (20 × 256) to a message or video in just a matter of seconds. Importantly, these 5,000 recipients could each in turn expose the message twenty times to a group of 256 people. Now we have reached 2.5 million people, and we haven’t even begun to consider friends of friends and their friends. — Sander van der Linden, Foolproof, 334

Categorization

• SHARE: Nerdy numbers about the 256 cap and 20-forward limit compounding to 2.5 million in two hops — a sharp concrete stat for anyone following platform design or misinformation.
• corroborated: 2 independent passages support this claim — Sander van der Linden, Foolproof: Why Misinformation Infects…, 334
• XREF: connects to: Viral WhatsApp rumors triggered fatal mob lynchings across India — Sander van der Linden, Foolproof: Why Misinformation Infects…, 320; Bolsonaro’s 2018 campaign bombarded millions with fake news via WhatsApp groups — Sander van der Linden, Foolproof: Why Misinformation Infects…, 336