I agree in part. The texting, social media, etc. has the same disjointed tone as the "now, this" attitude of television that Postman wrote about. However, he had a specific concern with images versus text (whatever kind of text) -- namely, that they pass too quickly for people to dwell upon and can never advance an argument in the way that writing can.
To be honest, I think this is a general weakness in Postman's work. He talks a bit about the difference between pre and post-telegram writing, but he lionizes all 18th century writing despite there being tons of different forms (pamphlets, books, novels) that don't have the same intellectual value imo.