I personally think that weight loss products are a very hard niche and can be a bit more difficult to promote in ways that you can use for other more ''general'' products. However, I still believe that there are effective promotional methods.
I wouldn't suggest using Facebook but instead, try to focus on perhaps creating a blog and a forum where you can easily discuss your products. You want people to feel ''safe'' when using them and there's no better way then with interaction. On your blog, you could easily post pictures of the products and tell the consumers what it's made out of, what they can realistically expect, any good success stories and more. The forum is there if they need any assistance or are curious about certain things.
Any sort of review pages will also do wonders for your promotion and sells. Let's face it - when it comes to weight loss products, we all love hearing the excellent feedback or how people lost XY pounds in a few weeks... it makes people want to at least try.
And lastly, have you considered using other social media platforms besides Facebook? There's plenty... Twitter, YouTube, Instagram and more.