> Exploiting the fact that you have "more users" should probably also be looked at as anti-competitive since it certainly doesn't benefit consumers by locking people into exclusive platforms.

It's not quite as clear cut as you say. The downsides of being locked into a single platform exist, but the upsides of a new feature benefiting from an existing network also exist. Many consumers have benefited by getting access to ephemeral messages with their existing friends, when they wouldn't have installed a separate app and nagged their friends to switch just to get that feature.