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 all their posts that they will get to them (have you ever seen the 'you are all caught up' prompt? I haven't even on some of my smaller following feeds). Once again engagement, and this time in terms of comments, will help increase your posts lifespan as Instagram may push the engaging posts to its Discovery page. Hashtags are also a well known way of being found at a later date, but ensure you have a mix of high volume and low volume hashtags.
LinkedIn has a reasonably good lifespan, sometimes 24hrs but can be seen up to a couple of weeks later. It is also affected by early engagement, and there is a rising usage of hashtags for discovery, with LinkedIn suggesting certain hashtags to users.
Pinterest has the longest lifespan (apart from blogs) with a range of a couple of months. Hashtag usage and of course strategic board usage can increase your chances of your posts being pinned and continuing to spread.
How will it affect your social media strategy? Of course, you don't just go for the longest lasting post platform, just because of the timing, you need to consider with each platform if your audience is on it or not. But these timings might be factored into your strategy in terms of how to make them last longer, or when your audience is available and your timeframe window.