It all comes down to what you are building and how your target audience will interact with it. Being mobile first is definitely hard when you are trying to penetrate into a sea of tough competition in the market especially if there is better use case to be web-first. For you, I feel you gained enough traction and very positive response by being mobile-first. I am sure that helped you get 1M or so users quickly on board and now that you want to go from 1M to 10M, you obviously need to think of entering new markets and platforms while maintaining your lead in mobile. This becomes hard and frustrating. We are a mobile-first startup trying to bring business cards onto mobile. It wouldnt make sense for us to be web-first, hence we are mobile-first.