That seems close-minded and overly judgmental. Snapchat might not have any use to you, but it's clearly the world to this girl (and many, many other teenagers).
The way it's described, I'd call that "flow" - she's snapchatting "in the zone." It's like responses in a lot of video games - you don't "see" the enemy / row of gems / hole for an L-shaped block, you perceive it and react to it. If you have to look, it's probably already too late.
This has to be one of the most ignorant interpretations of a social media product. "no relevancy to anyone, and is a waste of time and money." First, your "proof" of Snapchat's lack of relevancy is literally an anecdote of how it is relevant to a girl's life. Second, do you think that Snapchat has to have some productive, utilitarian use to not be a "waste of time and money?" Snapchat provides a service that users enjoy (and this user in particular adores) using. IMO, that's the end of the story.
ME: I’ve seen how fast you do these responses… How are you able to take in all that information so quickly?
BROOKE: I don’t really see what they send. I tap through so fast. It’s rapid fire.
I closed the tab at that moment. That alone proved Snapchat has no relevancy to anyone, and is a waste of time and money.