I am running a productized service making ~$10k in MRR called Manypixels (
https://www.manypixels.co) -- We have only been live for a few weeks and so far it has been content for us.
1. Facebook - I joined all niche Facebook groups about SaaS and startups (example: Paris Startups). I posted the following: "Would you guys find this useful? Yes? No? Why Not?", asked for feedback and that generated a lot of discussion.
2. Reddit - I posted a few learnings about how I launched this side project in 5 hours and got the few subscribers. r/entrepreneur is huge but competitive, r/startups is also huge but less competitive.
3. Other platforms: IH, Hacker News, Dribbble
Also works: Direct sales (mostly via Dribbble). When I will launch our v2 I am planning to add referrals (for our existing clients) & affiliate sales.
When we grow really big I will probably try Facebook Ads, or Youtube ads or Reddit ads but even though I am not so sure as I want to stay relatively small and high quality!