> My users are really happy with it, the conversion rate is good, and there's been a surprising amount of word of mouth from them already,

If all that is true, you already have hit the dreams of startup marketers. People post ads on various sites trying to get enough people that their users start word-of-mouth organic growth. What you need to continue growth is to focus even more on your current customers. If you burn your energy on sales, you risk ignoring them, and that organic growth might stop. So talk to your users more, focus on their experience, and make them so happy that they keep talking. Build the features they tell you are needed to make things even better. Ask them where they think other people would be responsive to marketing, and focus your efforts wherever they say.