Facebook and Instagram work fundamentally differently, as they can save a list of posts to your account rather easily. Remember, if you want it saved to your account here it has to be part of the blockchain. You could in theory still encrypt it to not be publicly available, but at the very least it would add a lot of additional information being saved for very little benefit. From a more realistic standpoint facebook etc probably only offer this feature to get more insight into your browsing habits to begin with.