It's not unusual at all imo for people of their age to not have a Facebook, and Twitter accounts really depend on the person. I don't have a Facebook account, and do have a Twitter but it only has nickname with no surname as the listed name, is private and has been updated maybe ~20 times in the 8 years I've had it, in times I've 'tried to get into' Twitter. It's just not for me though, at least in terms of posting my own thoughts - I do search for news etc sometimes, but only because I visit the homepage for work purposes. Neither GP or BL strike me as the typical Twitter user (although I'm willing to be wrong of course - just my opinion).
We do know Gabby had a Snapchat account - that makes sense to me, as a way to keep in touch and share life insights with close/approved friends. I'm a little older, 26, but I don't think it's unusual and I think young people are much less 'on' social media than older people think - in my experience, we're quite savvy about personal data and staying private with things that should be private. Even if people do have SM accounts, eg a TikTok or personal YouTube account, it may well just be for ease of viewing other people's content rather than sharing their own, and therefore isn't as publicly obvious or identifiable. All my humble opinion, of course
ETA: Of course Gabby, at least, was arguably trying to break into the world of social media influencing. I wasn't aware there was a TikTok account for her that had been tracked down, but it does make sense if it was related to her trip. All I meant to say was that from my experience, I wouldn't be surprised if anyone wasn't able to find public social media regarding their personal lives and insights, for example a Twitter account updated during high school, or a more personal account from recent years.
I'd not be surprised, for example, if Gabby had an account on Tumblr or similar blog site, but I'd also be surprised if she had linked that directly to her full name or any other easily-identifiable personal info. Hope that makes sense and that my original post still has some benefit/insight into the social media use of the younger generation!