Which social platform is best for promoting a SaaS product?
Short answer
The one where your users already are, which is often not the one founders default to. X and LinkedIn suit B2B and developer tools; Reddit and specialist forums suit anything with a passionate niche; TikTok and Instagram suit consumer products with visual before-and-after value; YouTube suits anything that needs demonstrating. The mistake is choosing by which platform you personally enjoy rather than by where the buying conversation happens.
The longer version
This question comes up inside a longer piece on the same problem. If the short answer above raised more questions than it settled, the full article has the reasoning behind it.
Every Way to Promote a Product, and Why Developers Get Each One Wrong