In fact, I found the answer to your question on the 4th page of their annual filing to shareholders[1]:
> While these metrics are determined based on what we believe to be reasonable estimates of our user base for the applicable period of measurement, there are inherent challenges in measuring how our products are used across large populations globally. For example, there may be individuals who have unauthorized or multiple Snapchat accounts, even though we forbid that in our Terms of Service and implement measures to detect and suppress that behavior. We have not determined the number of such multiple accounts.
It seems to me that Snap is quite open about their active user counts.