Here are the ways / tips to get backlinks through famous social media.
Facebook is very famous and there are tons of features that you can find. one example is a group, please join in a group that has to do with PBN / Backlinks, There Must Be Many Sellers offering their domains for backlinks, or you can start a discussion looking for backlinks including the details you want. There are also several Fan Pages that sell backlinks, you can search for them with keywords
Instagram is also no less famous, many Instagram users who use social media to sell / do business, one example is backlinks, you can find it by typing the hashtag that you want in the keyword example: #backlinks #needbacklinks