I'm starting to build my pitch deck and need more detail as we're launching a private photo messaging app.
The total addressable market is a function of the app's value proposition itself. Given that photos and messaging are now simply core features of pretty much any mobile-native application, you could cite every mobile user as your potential TAM.
Anyone telling you that TAM is a useful point for you to address in pitching early-stage mobile investors for this concept is giving you very bad advice.
What I would recommend you spend ALL of your time on is why you can dislodge users from the incumbents and why there is a specific unaddressed consumer pain that will gravitate them towards your app.
I'm happy to help you frame your pitch in a call but call or no call, TAM as a specific data point is not something to concern yourself with here.