I think seemingly sweet/innocent kids are often the ones who can have the most surprising online activity. Take it from me. When I was a teenager, I was shy, quiet, and a teacher's pet. My online activity would have shocked anyone in my real life circle - and this was before smartphones, back when forums and chats were primarily used. Still, my parents had no idea of my online activity. Today, it's even scarier, since there are hundreds of apps available for teens that adults don't know about. They're always coming up with something new, and it would be impossible for law enforcement to track some of that unless they're able to access every single device Jayme has used, and even then, it can be difficult.
Ultimately, I don't think her online activity is the reason for her parents' murder and her disappearance - I truly don't, because it rarely is the reason. It's always the first question asked, but it's rarely the case. Could she have been talking to someone online who was 19? Sure. Girls her age are often talking to other teens with similar interests, and she may have formed a close-knit relationship with someone on an app. What's scary is my boyfriend's 7-year-old niece is on some of these apps, and she has "friends" of all ages. Online dating/relationships are so frequent among teens these days. They're usually harmless (of course the select times they're not). Online activity rarely solves disappearance cases from the cases I've followed over the years.