As other suggested, forcing interoperability would go a long way (it should be noted that e.g. facebook messenger WAS interoperable when Facebook wasn't as large, but they stopped it when they became the leader).

Additionally, separate to instagram / facebook / whatsapp ; possibly also facebook social / facebook marketplace / facebook events ; They should have a level of access that others have. Imagine Yahoo users could subscribe to Facebook events without ever setting foot in facebook.com ... I know it sounds like science fiction, but it's possible. like email....