I believe it's fine to hide this information if the algorithm to rank posts is obvious and easy to reason about. For example, you know on Hacker News that the top posts are the ones with the most votes. More detail doesn't give much more information.
But Instagram is like an Ocean of content, there is no clear boundaries between topics, it's an endless stream of content and events happening in parallel, AND the algorithms are known to be opaque. So the information of number of Likes are valuable to relate to other events which happened in the past. This also applies to Twitter.
By the way, we can see the total votes on the front page of Hacker News, because it helps to put things in perspective according to past events.
Instagram should hide the numbers of like in messages in the discussion of picture, but IMHO they should keep at least the order of magnitude of Likes under the pictures. ("10s Likes", "100s Likes", "1k Likes", etc)