I enjoy the constraint and view it as a minor new art form, similar to the constraints of a Haiku. Seeing that '0' when I'm done is generally mildly pleasurable for me.
I almost exclusively follow stand up comedians on Twitter, and the character limit is perfect for a concise one line joke. I will actually avoid any "twitlonger" links or images of text, because that's not what I'm there to see. Likewise, I've unfollowed quite a few people on Instagram when they start regularly posting pictures of text. Maybe I'm just a format purist snob?
There's one comedian I know who deleted his twitter account (I think in part because of harassment he was getting, and also in an attempt to obscure his less savory posts from his non-famous days). Then he took to Instagram and started posting images of text with comments disabled. At that point, why even bother? Does the number next to the heart button mean that much to you?
Especially with Instagram, you can write as long of a comment as you want. Post a picture of some random thing and then write your long-ass comment below it.
Great, but how many people online read, write or enjoy writing Haiku? If writing a Tweet requires as much time and (intellectual) energy as a Haiku, than I'm not surprised that the growth of Twitter is tanking.