If retention and monetization rates are healthy, and you want to focus on acquisition, I would go with the following:
- Niche or vertical focused landing pages on your website, to build and grow a list of emails/phone numbers for your sales team.
- An Adwords search campaign targeted at your competitors, to try and steal some of their not-so-satisfied customers.
- Social. This is an obvious one, be on the social networks your customers spend the most time on, and engage them with valuable content (Do not try to sell them anything through social, at least, not at this stage).
- Offline marketing if you have the budget to do that, if not, focus on local barter deals and partnerships.
Best of luck :)