It depends on your niche. For general niche, facebook is far better than others. Just to give you a hint, these are the niches/topics that frequently find on social media sites.
Facebook - News, Entertainment, Offers
Twitter - Updates, Viral content, Reviews
Reddit - Memes, Jokes, Q & A, Support
Pinterest - Jewelry, Shopping, Fashion, Lifestyle
Instagram - Photography, Lifestyle, Offers
He said for "SEO strategy" in the OP, and if I'm not mistaken, most of IG is not indexed. Have you have different experience?
Facebook, Instagram, LinkedIn and Pinterest are the best social networks.