A popular way of gaining some traction in the SaaS world is to own a high intent keyword and then spreading from there. Social Networks must work similarly with owning one specific audience. Think super niche, like a kitchen appliance review community or something like that. If you can own a community you can own another. Marketing is basically like shouting into the void because new users will feel compelled by existing users but if there are none, the value prop of the app is weaker.

This, but I'd go even further. Wny not test run it by developing a hyper-local market first? You could make QR codes for neighborhoods and paste them up on telephone poles. Have a photo element to encourage people to post pictures of themselves there, or something they're seeing. Encourage people to make their own QR codes as well. Userland QR codes for local bands and pubs, etc. Not just products. Once the ball gets rolling in one town, who knows?