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.
Great question - to build also on Tom's response, I would say that the TAM for entrepreneurs who are building pitch decks is the start down the path of what I like to call "CEO math" - starting with TAM, while making logical sense, is really a means for lazy calculations on projecting market growth and even in crafting your go-to-market strategy. This is where people fall into the all too common "we will just get 1% of the market...." pitch; and that is a soundbyte that few if any serious investors will be welcoming.
It's an OK data point to quick reference, but if you are taking in terms of building out a market growth model and/or projecting finances based on assumed downloads, traction, MAUs etc, I would hyper-fractionalize your markets and go after a very narrow and specific segment (it's better to get 99% of a grape than 1 % of a watermelon.) Investors tend to respond more positively and can more easily wrap their minds around your model and plan if you show are you aiming down the sights a bit rather than shooting from the hip.
I'd be happy to help walk through data points, how to find them, and how to present them to potential investors in your deck. Feel free to request a call and we can go from there!