When did Twitter hit it's viral growth curve? And what was the user count before and after? To be clear, it's not necessarily the case that a platform needs to

When did Twitter hit it's viral growth curve? And what was the user count before and after? To be clear, it's not necessarily the case that a platform needs to optimize for growth, but I wonder what can be expected without the sticky features that "addict" the most users to a given platform.