Regular Facebook likes do not decrease with time. Because you will get it from the people who liked your post by liking your content and you do not pay them for this. These are your real fans and followers. This type of likes is your novel audience, once they like your post, it will not decrease over time. But if you bought them and their support or these are not your regular likes, this will decrease over time which Facebook will usually ban after some time because most of them are fake accounts.