Social media is probably the best channel available, with reasonable returns. However, "free" may be a false economy, as it takes a lot of time/effort to do it effectively. Pick 1 or 2 relevant social media platforms (Twitter & Facebook?) and put your effort there.
A lot depends on who your target market is, and what level of reach you need. E.g., if you need to a very specific set of users/customers, then make a list of your targets and try and reach out to the individuals on social media. If you have a wider set of users/customers, reach out to specific hashtags/keywords.
Alternatively, put some money towards Google AdWords, LinkedIn ads or Facebook ads. Go for the "long tail" of keywords, which should be fairly cheap.
Let me know if you need more detail around this.