WhatsApp doesn’t want to be a platform. Co-founder Brian Acton, on a panel Wednesday at Facebook’s F8 developer conference, made that very clear. Unlike its sibling service Messenger, which has started courting outside developers and businesses, all that matters to WhatsApp is that the service remain stable, simple and unfettered for its worldwide audience of 100 million monthly active users.That matters to parent company Facebook too, but likely for different reasons.