I disagree with the notion that Facebook is for the 'fogies'.
Like others, we have a closed Facebook group (for reminders to cadets/parents of upcoming events, tagging cadets in specific information, sharing photos etc), a Facebook page (for publicity), Twitter page and Instagram, which we use alongside email.
I would say the vast majority of our cadets use Facebook. Only one or two cadets (out of 30-odd) aren't on Facebook, and that may well be age-related. Hardly any (as far as I can see) use Twitter, and probably a little less than half use Instagram.
The reason for Twitter falling out of favour may well be down to the lack of characters allowed in posts. I have zero idea about Instagram, but as all members have the ability to post in the closed group and there are no limits on post size/attachments, Facebook works best for us, and as far as I am aware, for most squadrons I know of.
Through work I am internet safety & exploitation trained so ours get regular internet/social media lessons to ensure they are aware of the dangers & what to do to keep their profiles private. This may have an impact on the social media sites our cadets use.
IMO Snapchat is a totally inappropriate platform for Sqn use. What'sApp (a messaging service rather than SM) is also inappropriate and can not be monitored if people go outside the group (as those in groups will have each other's mobile numbers). As I say, most kids don't use Twitter now so that just leaves Instagram & Facebook as the other main social media platforms.