Facebook. Big surprise. 2.6 billion active users. They have funny rules and are definitely in business to be in business. Posting for promotion here without violating their community rules is an art form.
Instagram. The younger, more visually-oriented sister of Facebook. Without the video and color of Instagram, you could never have had the naughtier pay platforms such as OnlyFans.
Twitter. They have relaxed their content rules so that adult-oriented material can play within certain limits. However political the content tends to be, they are conscientious about not losing the republic to psychopaths. Promoting your business here, as with Facebook, is an art that can often get your account shut down if not performed with care.
LinkedIn. These folks are amazing. With 650 million active users, the only issue is that everyone posts and very few read. Nevertheless, any valid social media campaign requires LinkedIn if only for legitimacy. If you exist, you are on LinkedIn.
Tumblr. If your product or service has any value to teens and 20s, you need to make occasional drops on Tumblr.
SnapChat and WhatsApp are fun, perhaps, but the value of promotion of tangibles here is negligible.
Pinterest. This scrapbooking platform has untapped power. People who have a routine of visiting Pinterest are serious folks who trust the value of this medium.
YouTube. If you try to run unpaid ads here under the guise of promotion, you may get bounced. If what you are selling is promoted as strictly informational or entertainment, you can get a strong audience as long as you cross-promoted from other platforms.
Mix. When this was StumbleUpon, it was fantastic and drove traffic like a mofo. Nowadays it is mostly a boring news aggregate.
Nextdoor. For tangible products or legitimate services with a local focus, NextDoor is a must. The larger your community, the larger your reach.
DeviantArt. As with LinkedIn, this site for artists is more about posting and less about viewing. However, it remains a beautiful platform and any serious artist should make an appearance here.
MeetUp. This allows people to connect with local groups to meet new people.