Where can you advertise for free?

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.

I've gone through a lot of this myself. Pumping money into social network advertising is not (or highly unlikely) to return a strong ROI but it depends on your product and who you want to reach.
It is much more effective when your brand presence has grown enough to create a Halo around your advertising; right at the start, you are going to burn a lot of cash for little results.
The best thing to do is to get the basics right first. Create multiple touchpoint for your business with social media pages (it looks like you have that). Make sure you have a few videos on YouTube, this counts toward SEO. Be active with blogging and content engagement. Publish on LinkedIn, create a blog and start engaging your audience and drive traffic to your site.
If your business wants B2B advertising, check www.arlians.com; it's in BETA but if you sign up now, you can get some level of advertising access for Free (for life)

Hello, I think you should have asked first instead for what platform gives the best ROI. More specifically for what purpose?

Is not just about saving money, what about your time? Do you have the right concept in place so that you're not interrupting viewer's time with random ads that they are not interested in? Potentially costing you sales rather than driving them.

For example if you are promoting a new beard oil for hipsters, you are probably better off shying away from FB. While Instagram or Twitter might be better for ROI.

Reddit, hmm...it is a SaaS? otherwise maybe not so much.

For example,I marketed a pair of crotchless shorts for outdoor enthusiasts. Particularly females.Anyway for this product, a combination of paid google searches, instagram and facebook proved to be a great combination.

The ads, however pointed to custom landing pages I created with A/B testing to see what worked best. Tweaked as needed and re-ran. We went from 0 to over 9,000 monthly visits after creating a brand new domain for it w/ zero prior SEO.

In Phoenix, my wife is the owner of Honest Maids. I started off the marketing for this the same way. But I used Instagram, facebook and business cards customized for events and each with a unique purpose link. - to a landing page. Instead of waiting the typical 3-4 month wait for clients to call in for services I did it in less than a month.

For our marketing and web design team I use a combination that includes Linkedin, Craigslist, Quora, Twitter, Referral systems and blogs' backlinks.

(obviously me answering here for you is another way of adversiting something for me...)

It all depends on where your clients are... follow them not the price to advertise there ;)

I believe that social media is the best place to advertise your product for free, but since you have already tried it, you can try online directories for your business.
Ranging from high-profile platforms to local listings, online directories cover a range of audiences. The method for each of them is quite similar: you submit the URL of your professional website, as well as additional details about your business or organization. These sites, in turn, incorporate this information into their data banks, ensuring that a link to your site appears when users search relevant content categories. In addition to displaying your business name and website, these free advertising sites can improve your search engine optimization, or SEO.
You can read more here: https://www.wix.com/blog/2016/12/free-places-to-promote-your-website
Besides if you do have any questions give me a call: https://clarity.fm/joy-brotonath