Instagram's tagging system was actually really effective at categorizing content and discovery because each hashtag was treated as a node in a (giant) graph, where each node has multiple properties, including post count (number of posts using a tag), 'velocity' (number of posts using a particular tag per unit time), etc. I could write up a big post about it as I made a study of it in when I created a web app for finding the most relevant tags a few years ago.
All that to say there was a lot to their system and it worked because users became aware that they were rewarded for using the most relevant tags. Using irrelevant tags was punished. This guided users towards using a mix of relevant popular and niche tags to maximize their reach, which, in turn, further improved the tagging system.
Instagram's tagging system isn't as important anymore as their algorithm has deemphasized it, in favor of other methods for classification and discovery, but there were a couple of golden years where it worked very well. Most users still look back on those years as the 'good times' even if they don't know exactly why. I'd go so far as to say they ruined the app after they deemphasized tags (and added way too many ads)
All that to say there was a lot to their system and it worked because users became aware that they were rewarded for using the most relevant tags. Using irrelevant tags was punished. This guided users towards using a mix of relevant popular and niche tags to maximize their reach, which, in turn, further improved the tagging system.
Instagram's tagging system isn't as important anymore as their algorithm has deemphasized it, in favor of other methods for classification and discovery, but there were a couple of golden years where it worked very well. Most users still look back on those years as the 'good times' even if they don't know exactly why. I'd go so far as to say they ruined the app after they deemphasized tags (and added way too many ads)