How to reinvest earnings for long term growth?

Last year I co-founded Volca.io which is a development platform for building production ready SaaS products.

Since me and my co-founders have kept our day jobs we are thinking about how to reinvest our earnings for long term growth so that we at some point can work on Volca full time.

One type of service that I have seen is the one where your product is posted to various directories which I am considering purchasing. I have considered simply buying ads but I imagine that will require constant monitoring and tweaking to get right.

Do you have any other ideas on what I can spend on to get more traffic and more customers?