I had no idea where to ask such a question and I tried to read their terms of use / privacy statements / about us page to find info about how this popular cost splitting app makes money to employ people.
They don’t even accept donations. Sounds very sketchy, but apparently they don’t really sell data or even offer this app to businesses. Does anyone know how they make profit?
I’m curious what kind of data they are selling.
Like Joe always default on his payments? Or Nancy occasionally let Lucy pay the bill. 
Write them and ask
What kind of things people buy would be my guess. Then advertisers can add that to whatever profile they have on you so they can give you ads of products that fit that profile.
I’m never surprised by what data people sell.
I’m always surprised there’s someone buying.