I've seen a couple of radio stations who have Snapchat for their morning shows and you can add them and see funny photos or get sneak peeks into upcoming contests on the radio. I think I would create a separate Snapchat account for the library instead of using my personal one if I were going to do this. I could send Snaps to patrons with "new arrival" movies and books. I definitely think kids and teens would enjoy this type of interaction.
Here's a recent Snapchat I received when my son's auntie was babysitting him!
As far as Instagram goes, I have my Instagram account connected to Twitter and Facebook, so that's how I share my personal photos after editing them. I definitely think there's a balance that needs to be kept as to how many different social networks the library uses. You want to cater to all of your patrons, but on the other hand, you only have a certain amount of time you can take to upload content to different networks. Apps like Hootsuite are helpful because you can link networks together and do one post, but it is still possible to stretch yourself too thin.
Here's one of my all-time favorite photos taken and edited with Instagram. This is what I saw from the circulation desk last summer: