Hey Suz,
Snapchat is a private app - only people you accept can see your shared photos and you choose who to share them with. The picture of Abigail at 2pm would've been uploaded to Libby onto her 'Story' - a feature of snapchat that allows users to share elements of their day in a viewing platform as such. It can only be viewed by approved friends. (People Libby herself had approved on her own account) Snapchat also SHOWS you who has viewed your photos. You can see usernames. I'd assume if they had a Facebook or Instagram that they would've had it set to private too. Also, most teens (from my experience anyway of having a lil' sister who is 14) don't tend to share where they're going or what they're doing until they're actually right in the middle of it. It just seems unlikely that the killer stumbled across a photo or check-in that afternoon and made his way there. He knew the area far too well. I think the whole social media (or snapchat in particular) giving away where they were thing is almost certainly not how this creepo found these girls. It's likely he had planned to kill that day and they were the most vulnerable targets OR he had been following them around days prior to the murder.
[[Also, yes your snapchat story can be on public BUT someone would have to know your username to add you and you would still 100% know that they are viewing your photos on there. If he had been lurking via social media prior to the killings it was not on snapchat!]]