The combination of thinking about exactly what one wants to say via text, and then being rewarded (or punished) by an instant reply is very hard to resist.
I blame the mobile Internet as a whole, and image and video sharing sites in particular. Modern social media is just a next iteration, but ever before them, people of all ages got hooked browsing sites with user-generated memes (mostly "demotivators" back then). Intermittent reward only properly kicks in once you have unlimited access to an endless stream of media that you can evaluate and consume in seconds.
Jup, and just like every other next iteration, it also increases the attached problems. Up to the point it cannot/shouldn't be ignored anymore.
(Not really targeted at you, but I've seen the "nothing really changed" argument seen used way too often to support "we don't have to do anything" as to not trigger me)
Why are you so opposed to it?
Yes, IM does provide a lot of value and most people will not become addicted. Yes, other smartphone uses are certainly more problematic for most people. But searching for "SMS addiction" yields quite a bunch of hits, with a lot of them from before the smartphone era. This indicates it to be indeed a problem for some people.
Each case in the self help group for excessive screen based media consumption I visit seemed to have its own unique flavor. Don't think there was anyone with a heavy and exclusive focus on IM among them, yet. Probably cause the audience of this particular program is mostly male. There probably also aren't that much such cases in general. But I can totally see this being the addictive "substance" for some. And heavy users should watch out for detrimental effects on other parts of their life.