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. 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. 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