Networking over Instagram and Twitter and asking users to try the product has worked for my client in the past very well.
Marketing cost = ZERO.
What technique has worked for you guys?
It depends on the niche. Building a personal brand on Linkedin for a B2B-oriented software is great.
On the other hand, if your niche is B2C then targeting those communities, Pinterest is really good for generating organic traffic. If you connect it with a good landing page and funnel, you can generate subscribers and leads.