The reason being, the users actually don't use the save/heart button after the post.
Say for yourself, how many times you actually went above and clicked save/heart button after reading comment or clicking additional links? Or watching that big footer?
Well, maybe people shares the post even at that point, but honestly this feature is very new and monitoring the behavior might show more UX hacks in future.
You make a very good point here. And I guess this might be obvious enough for users to actually "get it" the first time they see it.
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