Glad to hear you've been finding Overleaf useful!

Re the business model: we don't rely on the user subscriptions for revenue -- we provide services for institutions (e.g. Overleaf Commons: https://www.overleaf.com/universities) and publishers (e.g. templates and direct submission links: https://www.overleaf.com/publishers#!publisherslist), as well as private cloud / local installs for companies: https://www.overleaf.com/enterprises

(and that allows us to provide a solid free plan that works well for most users, especially those just starting out with LaTeX)

But yes, the "how does it make money?" question was always one we found difficult to articulate well -- we certainly didn't do it in the YC interview! :)