Social Media Posts only display in a person's newsfeeds for a certain amount of time, due to the proliferation of posts. They can vary depending upon each platform but also can vary due to the reactions, comments, pins, saves and shares that they get.
This infographic explains the average time on each of these main platforms and some hints on how you can make your posts last longer.
Facebook is the shortest timeframe of 2 - 5 hours - probably because of the popularity of Facebook, with only Twitter with a shorter timeframe (15 min). With Facebook try to get engagement in the first 15 minutes - 1 hour after you post, as the algorithms feed your post out to some of your followers and based on their reactions will feed it out to more and more (to your organic reach).
Instagram has a timespan of 21- 48 hours, in fact your posts are still there but it is only if people scroll through...